Okay, so I'm thinking of buying the 21.5" i5 iMac for £999 http://store.apple.com/uk/browse/home/shop_mac/family/imac/select?mco=MjIwNTQyNjE.

It has these specs:
2.5GHz Quad-Core Intel Core i5
1920 x 1080 resolution
4GB (two 2GB) memory
500GB hard drive1
AMD Radeon HD 6750M with 512MB

Can anyone give me an idea of how much an ordinary windows PC with these sort of specs would cost? (Obviously it would only be a very rough estimate). Thanks

With £999, you can get an self-built PC with i7-2600k, a good aluminum case, 8GB of RAM, a solid state drive + storage drive, a good graphics card, and a good 24" 1080p monitor.

Just to let you know, the HD 6750M is actually a mobile GPU. Mobile GPUs are much weaker than their desktop counterparts.

Edit: I just checked prices in UK and... my god, they're ridiculous.
Anyway, for a self-built with better specs:

i5-2500k
1920x1080 24" LED Monitor
MSI P67-G45
8GB DDR3-1600 CL9 1.5V
1TB Samsung Spinpoint F3
HD 6950 2GB
XFX 650W power supply

CPU - £155
http://www.aria.co.uk/SuperSpecials/Other+products/Intel+Core+i5-2500K+3.30GHz+%28Sandybridge%29+Socket+LGA1155+Processor+-+Retail+?productId=43216
GPU - £203
http://www.aria.co.uk/Products/Components/Graphics+Cards/ATI/ATI+6900+Series/MSI+ATI+Radeon+HD+6950+Twin+FrozR+II+OC+2048MB+GDDR5+PCI-Express+Graphics+Card+w%2F+FREE+Deus+Ex+Game%21+?productId=43820
Motherboard - £110
http://www.aria.co.uk/Products/Components/Motherboards/Intel+1155+P67+%28B3%29/MSI+P67A-G45+Intel+P67+%28REV+B3%29+Socket+1155+DDR3+PCI-Express+Motherboard+?productId=44075
RAM - £44.39
http://www.aria.co.uk/Products/Components/Memory/DDR3/Dual+Channel+1600MHz/8GB+G.Skill+RipJawsX+%282x4GB%29+DDR3+1600MHz+9-9-9-24+Dual+Channel+Kit+-+F3-12800CL9D-8GBXL+?productId=46060
Case - £84
http://www.aria.co.uk/SuperSpecials/Other+products/Lian+Li+PC-60FNWB+Midi+Tower+Aluminium+Chassis+?productId=46000HDD
HDD - £40
http://www.aria.co.uk/Products/Components/Hard+Drives/SATA+%28Serial+ATA%29/500GB+to+1TB/1TB+Samsung+HD103SJ+SpinPoint+F3+SATA-II+3.5%22+Hard+Drive+?productId=37726
Monitor - £150 http://www.aria.co.uk/Products/Monitors/24%26quot%3B+Monitors/24%22+Iiyama+ProLite+E2407HDS+Widescreen+LCD+Monitor+-+Black+?productId=34750
Power Supply - £65
http://www.aria.co.uk/Products/Components/Power+Supplies/650W+or+less/650W+XFX+Core+Edition+80PLUS+Bronze+Power+Supply+?productId=45333

Total is ~£812 so you have ~£190 to spend on mouse, keyboard, speakers, etc.

With £999, you can get an self-built PC with i7-2600k, a good aluminum case, 8GB of RAM, a solid state drive + storage drive, a good graphics card, and a good 24" 1080p monitor.

Just to let you know, the HD 6750M is actually a mobile GPU. Mobile GPUs are much weaker than their desktop counterparts.

Edit: I just checked prices in UK and... my god, they're ridiculous.
Anyway, for a self-built with better specs:

i5-2500k
1920x1080 24" LED Monitor
MSI P67-G45
8GB DDR3-1600 CL9 1.5V
1TB Samsung Spinpoint F3
HD 6950 2GB
XFX 650W power supply

CPU - £155
http://www.aria.co.uk/SuperSpecials/Other+products/Intel+Core+i5-2500K+3.30GHz+%28Sandybridge%29+Socket+LGA1155+Processor+-+Retail+?productId=43216
GPU - £203
http://www.aria.co.uk/Products/Components/Graphics+Cards/ATI/ATI+6900+Series/MSI+ATI+Radeon+HD+6950+Twin+FrozR+II+OC+2048MB+GDDR5+PCI-Express+Graphics+Card+w%2F+FREE+Deus+Ex+Game%21+?productId=43820
Motherboard - £110
http://www.aria.co.uk/Products/Components/Motherboards/Intel+1155+P67+%28B3%29/MSI+P67A-G45+Intel+P67+%28REV+B3%29+Socket+1155+DDR3+PCI-Express+Motherboard+?productId=44075
RAM - £44.39
http://www.aria.co.uk/Products/Components/Memory/DDR3/Dual+Channel+1600MHz/8GB+G.Skill+RipJawsX+%282x4GB%29+DDR3+1600MHz+9-9-9-24+Dual+Channel+Kit+-+F3-12800CL9D-8GBXL+?productId=46060
Case - £84
http://www.aria.co.uk/SuperSpecials/Other+products/Lian+Li+PC-60FNWB+Midi+Tower+Aluminium+Chassis+?productId=46000HDD
HDD - £40
http://www.aria.co.uk/Products/Components/Hard+Drives/SATA+%28Serial+ATA%29/500GB+to+1TB/1TB+Samsung+HD103SJ+SpinPoint+F3+SATA-II+3.5%22+Hard+Drive+?productId=37726
Monitor - £150 http://www.aria.co.uk/Products/Monitors/24%26quot%3B+Monitors/24%22+Iiyama+ProLite+E2407HDS+Widescreen+LCD+Monitor+-+Black+?productId=34750
Power Supply - £65
http://www.aria.co.uk/Products/Components/Power+Supplies/650W+or+less/650W+XFX+Core+Edition+80PLUS+Bronze+Power+Supply+?productId=45333

Total is ~£812 so you have ~£190 to spend on mouse, keyboard, speakers, etc.

The nvidia Gtx 580 alone costs about $500, the motherboard, cpu, psu unit, ram, hdd, ssd and the case will cost you about $1200.

Dont't buy a ready-built desktop from a known company, like hp or dell, because they're very hard to upgrade and in most cases, impossible to upgrade, just get you self an asus sabertooth p87 motherboard and install the parts "Colourful" mentioned. But you have to make sure that those parts fit the motherboard, check youtube, there's a channel called thenewboston, they explain everything you need to know about building your own desktop.

Hang on there. What do you want the computer for? With Mac you doom gaming. Mac's are only better than Windows for multimedia and stuff with videos/music download/playing/etc..

With 900 pounds you can get a whole lot better Windows computer with at least:
i7 2600k 3.3GHz
8GB DDR3 1600MHz RAM
Radeon HD 6950 2GB
and keep going...

You can get a Dell 21" all-in-one for £399 but a Mac is more than its component parts so there is no real direct comparison.

£900 with better spec

you could get intel i7 2600k
8gb ram 1600mhz
gtx 580
750 power
80gb ssd
1tb hard drive
if you get windows

£900 with better spec

you could get intel i7 2600k
8gb ram 1600mhz
gtx 580
750 power
80gb ssd
1tb hard drive
if you get windows

The only Mac I like is Mac & Cheese

For £1000 you could buy or build a much better PC.

Dont buy any Mac they are overpriced.