PC game lag

[rusho00]

I have a Windows Vista home premium computer with an NIVIDA 8500GT graphics card with 256MB, an Intel Core 2 Quad Processor Q6600 each core operates at 2.4 GHz, 2 x 4 MB L2 cache, 1066 MHz FSB. I have a 500 GB SATA II hard drive. Dual channel 2048 MB DDR2 memory, Memory Card Reader 15-in-1. I have a 32-bit operating system and 2030 MB of rams. My problem is, however, that I have constant game lag while playing all next gen gaming titles (offline). Games such as Bioshock, Jericho, Call of Duty 4, ect. I am not able to play these games on the highest graphic settings without lag, (sometimes not even fair graphic settings). I don't understand why.

[MadKad]

So your just playing offline?

or online with other members on a server?

also do you keep other programs running in the background?