My PC is one year old and at the start I was able to play games on higher settings than I can now (same games d0h). For example I was able to play DiRT on Ultra High settings with no problem, but now I can only play it on Medium settings. I have reinstalled windows (XP 32bit) one week ago (and ofcourse several times in that year from when I have my PC). What seems to be the problem (what can I do to get the "original" power/performance ?

My PC:

Intel Core 2 Quad Q6600 @ 2,4GHz
2GB of DDR2 RAM - Geil Long DIMM
256MB XFX Geforce 8600GTS
400GB WD Caviar HDD
Running XP 32-bit

This games below are must play for everyone: 

Half-Life 2
Half-Life 2 Episode 1
Half-Life 2 Episode 2
BioShock
Gears of War 
Dead Space 

And if the old graphics doesn't bother you, you should also play Operation FlashPoint.

I wanna buy another 2GB of DDR2 RAM and I'm wondering if it'll have any effects on performance of my computer (in games, programs...). Curently i have this rig:

-Core 2 Quad Q6600 @ 2,4GHz

-2x1GB of DDR2 Geil LONG Dimm RAM

-Asus P5k

-XFX Geforce 8600GTS

And i have also one questin. If i'll run virtual machine on 32-bit XP and I place 750MB of RAM on virtual machine, will it take ram from 3,25GB that 32-bit XP's supporting or will it take it from my 4GB total RAM.