I'm trying to build a gaming pc on an extremely tight budget and I don't own a desktop to upgrade. Rather than buying everything separately, I think it might be cheaper for me to buy a prebuilt machine with an i3 CPU then adding a low end GPU and upgrading the RAM. Is this a dumb idea, and what problems could I run into? I'm aiming to run games like GW2 on a 1440 x 900 monitor with medium settings at around 30 fps.
Another reason for doing this is that it'll take care of all the bits like PSU, wfi, hard drive, operating system, optical drive, card reader, case etc that seem to cost significantly more if I buy them separately and that I'm bound to forget one of.
My budget is absolute max £400, preferably a lot nearer £300, less would be amazing. It's an extremely tight budget, I'm a student I need my money for things like books and food, so I'm looking at saving as much money as possible anywhere and anyway I can. It's quite likely that it's impossible, in which case I'll just have to wait till I graduate and get a job.
(I don't really mind paying more in the future when upgrading, spending as little as possible now is my biggest concern)
You would be better off building a budget pc with an eye towards upgrading and skip the gaming for now. Buy a good mobo with integrated graphics, processor and ram with a PSU that will accept a powerful graphics card. In the future, plan on spending $300+ USD on a GPU.
Well I found a computer that was basically exactly what I was looking for on eBay so I bought it quickly before someone else did. i3 2100 (not the best, but I'm pretty sure it'll do), 300Gb hard drive (more than enough for a few games, all I'm using it for), 4Gb RAM (if it's a single card I'll use it, if it's two 2Gb cards I'll replace one or both with 4Gb cards) and Windows 7, for £170. Leaves me £230 for a keyboard and mouse, some RAM and a GPU. I think I might be able to get it all for under £300, yay!
 @egilbe, gaming is the only reason I'm doing this, I have a laptop that handles most stuff extremely well, but it's integrated graphics can't handle the games I want to play.
Wow, that's a good deal. I've never seen that site before, I'll check it out, thanks. I'm not sure I want to spend that much on a GPU at the moment though, I don't think I'd see the benefit over a card that's like half the price when my monitor is only 1440 x 900.
Done! Final specs are: i3 processor, 8Gb RAM, Radeon 6870 GPU. Total cost (not including stuff like mouse/keyboard/screen or postage) was £300. Even with every possible in game graphics setting maxed out, on a 1440x900 screen I get 100+ fps in Counter Strike: Global Offensive and 30-40 fps in Guild Wars 2. Mission successful! Thanks everyone for your help, I really appreciate it.