Reccomend Me A Card!

Hi guys,

I just bought a Dell Vostro 200 PC which I managed to get at a really awesome price.

The only thing is, I want to play games such as Call Of Duty 4 on it, and the integrated graphics card just won't handle that. I'd like to buy a new card which would give me good performance, but I'm wary of buying some crazily advanced one and then realising my psu can't handle it!

Here's my current PC specs:

Intel Core2Duo E4500 @ 2.2GHZ
2037MB Ram (I've got another 2gb in the post right now)
Intel® Graphics Media Accelerator 3100
300 Watt DC Power Supply

I think that's all the relevant information, tell me if I need to give any more...

I'm not particularly experienced at buying computer parts to be honest, so it's a bit of a grey area for me, I know there are lots of factors to take into consideration when buying a new card, it's just finding somewhere which is gonna give me all that information which is the issue!

Thanks guys!