9800gtx+ problems running games

Just got a new system. Have spent two weeks trying to solve this. Several newer games (e.g. Comp. of Heroes, World in Conflict, Colonization IV: Civilization) are constantly freezing up or going into a sound loop with a frozen screen. I have tried all series of driver from 175 up to 178.13. 178.13 was the worst. I have updated all chipsets, drivers, etc. and have run memtest, etc. I used driver sweeper between installs. I can run FEAR, COD 2, and Brothers in Arms so far with no problems. I have tried lowering the resolution, turned AA off, etc. I am pulling my hair out b/c what is the point of paying for a higher-end card if I can't even run games, to say nothing of cranking up the resolution, effects, etc. I am running XP 32-bit Home Edition, SP3. I have nothing overclocked. My fan has the video card running at ~45C idle, ~55C or so under load. Would love any help. Here are my system specs:

-BFG TECH, BFGR800WPSU 800W Power Supply, 24-pin ATX12V EPS12V, Quad +12V,
-GIGABYTE, GA-EP45-DS3R, LGA775, Intel® P45, 1600MHz FSB, DDR2-1200 16GB /4, PCIe x16 CF /2, SATA 3Gb/s RAID 5 /6, HDA, GbLAN /2, FW /3, ATX,
-INTEL, Core™ 2 Duo E8400 Dual-Core, 3.0GHz, 1333MHz FSB, 6MB L2 Cache, 45nm, 65W, EM64T EIST VT XD,
-ARCTIC COOLING, Freezer 7 Pro Quiet CPU Cooling Fan/Heatsink, Socket 775, Ceramic Bearing,
-CORSAIR, 2GB (2 x 1GB) XMS2 Xtreme PC2-6400 DDR2 800MHz CL4 (4-4-4-12) 2.1V SDRAM 240-pin DIMM, Non-ECC, w/ Heat Spreader
-eVGA, e-GeForce® 9800 GTX+ 738MHz, 512MB GDDR3 2200MHz, PCIe x16 SLI, DVI /2, HDTV-Out,
-HITACHI, 500GB Deskstar® T7K500, SATA 3 Gb/s, 7200-RPM, 16MB cache
-SONY, CRX320EE Black 52x32x52-16A DVD-ROM/CD-RW Combo Drive, EIDE/ATAPI,
-SAMSUNG, Super-WriteMaster™ SH-S223 Black 22x DVD±R/RW Dual-Layer Burner, SATA, w/ Software, OEM