Need help with PC upgrade

[PVT.Roger]

Hope it's o.k. to post this. I haven't done a PC upgrade, or for that matter done much gaming in quite awhile. But with CoD it looks like the time is right. I'm a novide when it comes to these things, but know that a lot of you guys are experts, so will really appreciate your help.

Below is what I have now. I have $1,500 dollars (US) to use for an upgrade. Suggestions on the best use of that are what I'm looking for.

Intel Core 2 Duo CPU E8500 @ 3.16 GHZ
NVIDIA GeForce 9800 GTX
4 Gig Corsair Ram

Thanks in advance for any and all help. I don't worry about insulting my system, I know it's old.

PvtRoger