Half life 2 problems(flickering, CTD )

The flickering occurs from the title screen through out the game in random places (people , walls, objects, the sky) adn what happens is that al color will be lost or the texture disappears completely then comes back (sometimes not so fast on people where it can white out their whole body and all you see is the shape of them walking around talking). Also (and this is wrost part) whenever the game gets processing intensive like in an outdoor fire fight, or when i'm really playing around with the physics engine, even sometimes loading to the next area, the sound starts stuttering and the screen freezes followed by a crash to desktop with the meesage of "Instruction at reference memory blah blah blah could not read memory". I ran speedfan to check my temp and the cpu is at a rocksteady 48 C. I have 5 system fans and an thermaltake volcano 11 and my system temp is at 22 C. I do not overclock, but I checked cpuz to see my system settings and it did not recognize one of my ram sticks in slot #1 but it still said I had a gig of ram. So I took out the ram in slot 2( the recognizable ram) and ran cpuz again and this time it did not see any ram in both slots though it recognized that I had 512 in my system. So I swapped the rams poition ( I only have 2 dimm slots) and now cpu z recognizes both. But that still did not fix the problem. So I ran Prime 95 and on the torture test with the blend ram intensive option and it found a hardware error in 1 minute ( on the 1st low ram intensive torture test option, no errors were found). So THEN i ran memtest on boot up and it found 4 errors in 6 tests over a 3 1/2 hour period ( I stopped it short of its 10 tests regime). Could it be my ram is causing the mem problem and the flickering? I have used the individual sticks separately in my mobo in both slots and it still acts the same.
I also have this flickering problem with UT2004. Doom runs fine at high quality and actualy half life 2 runs fine as well (although lowering the settings well below my recommend ones still does not solve and problems). Could it be my mobo sloT #1 ?
I need this communities expert guidance please. Thank you. Oh, and all my drivers are up to date.
System Specs:
AMD Athlon Barton 3200 2.2ghz (clocked down to 1.9ghz because if put back to 400mhz fsb, I get weird color anomolies in some games )
Biostart/Amptron M7ncd Pro Mobo w/ nforce2 chipset 400fsb
2 kingston value 512 DDr ram pc3200
powercolor radeon 9800xt 256mb
1 wesern digital 80 Gb had
1 seagate 60 Gb had
1 dvd rom, 1 dvd/rw rom, 1 dvd/cdrw rom
1 controller card
1 generic 500 watt psu
soundblaster audigy2 gamer w/ 7.1 speakers
5 system fans, thermaltake volcano 11 heat sink
1 oz. of blood, sweat and tears
Winxp Pro SP2