Thats a really weak psu for a real graphics card. I wouldn't try running a gts 250 on that. It would be worth getting a new 450 or 500w and then a 250 would run fine. A 4850/4870 might work though. Also, what brand is your power supply?
so whats the most powerful graphics card i'm gonna be able to get thats suitable for a 300w power supply?
Wait until the end of this week, and ATI will have released its replacement for the 4670. The 5670 will offer substantially better performance for a similar power use. It will also support direct X 11.
The Nvidia option is a GTS 240--It is also more powerful than a 4670, but a little more expensive. If you were to get it the ddr5 version is faster.
seeming as it might be problem, how expensive/ easy is it to upgrade the PSU?
Very easy. Its a good idea to put in a high quality power supply anyway--because PSUs are the most likely component to fail in a computer--and if they do they can damage other components.
I upgraded mine to a Corsair 400cx for $39 after MIR.(newegg) It runs 82% efficient, the fan is completely silent, and it blows cool air out of my case. It also has the amperage to run most cards under $200. Corsair, Antec, OCZ, Seasonic are all quality PSU brands.
Luckily i found an old graphics card in my brother's desk he didn't want, is an ATI 512mb (can't remember the exact name atm) so that'll do for now. Think in the future i will definately upgrade the PSU then go for a more meaty graphics cards, i do like the sound of a silent PSU
Failing that i'll look into getting the ATI 5670 if it is that highly recommended and that much better than the one i have at the moment, i'll reply again when i get hold of the name and when i start tinkering with my pc more