Thinking about upgrading, but upgrading what?

[dass]

So, lately, I've been thinking about getting some new stuff for my PC once I get some cash because I wanted to play games like BF3 on much higher settings without laggish behavior, but I don't know exactly what should I change and honestly, I don't know what my budget would be, because I was planning on doing the upgrade after getting a job and some money, but, since I don't know when that will be, lets say 200-250€ is the budget.

Don't have a picture of the specs right now, but I know it by heart:

Asus P5Q Turbo motherboard
Core2Duo E8400 clocked at 3.60GHZ, was 3.00 (heats up like a bitch now, usually to, if its correct, 80, 90 or even 105ºC)
Nvidia GeForce 9600GT 512mb (made a very small clock to its mhz or whatever, can't remember exactly what I did, but I don't think it was that big of a deal, and this seems to be the part that really needs updating)
4GB RAM
320GB HDD (yes, I know, very small, I'd get another to add up to this one, like 1TB to make it an overkill of an upgrade compared to the old one, but I don't know if I want to sink all the money on something truly important)

That should be all... It can run Valve games perfectly right, CoD games mostly at max (except for BO because of optimization) and only has real trouble with games such as BC2 (and BF3 ofc)and both Crysis.

I could have had a better card then that nvidia and probably a phenom CPU but sadly, the website I used didn't have AMD products and the case ate alot of money.