I've looked at your specs and given how's it's equipped overall, I'd suggest something that isn't too over-the-top.
Your "P5LP-LE" motherboard has one PCI-E slot for a graphics card. I am guessing that you do not have a larger than 19" monitor.
Personally, I'd advise against getting any of the GTX-version of the 8000 or 9000 series of Nvidia cards, since they generate a lot of heat. Probably more than your (I'm assuming here) medium format case can handle.
A fine work-horse of a graphics card that supports modern-day games well, could be the Nvidia 9800 GT (512 Mb), however it does require a 500watt power supply (PSU). Going any "higher" in terms of performance, I think you will hit "a wall" in terms of cooling and performance, considering your processor, bus and RAM. AS it is, it's a toss-up and you might even have to add extra cooling with even this card - either place the case near an open window or use the PC with one of the side-panels off and a regular household-fan blowing into the case.
This card will run all the new games at reasonable frame-rates at 1200X1024 resolution.
Just keep in mind that this upgrade will only "tide you over" for a while. A full system replacement will be required within a matter of 1-2 years, if you want to be able to play latest-gen games even half-decently.
Also, you will need to add another Gb of RAM to avoid the worst "bottle necking" performance-wise with these games.
And then it becomes a question of whether it's best to:
- new power supply
- new graphics card
- another 1gb of RAM
and put it in ageing hardware, to tide you over for another year or so or
2) Buy another PC, customized for gaming from scratch
Good luck with your planning