Errors installing games, Nv4_display errors, artifacts weird colors/textures

[gamer4156]

So my set up right now is:
Mobo:Asus P5b Deluxe
graphics:Evga 9800Gt(had an 8800GTS 640mb but it died so I rma'd about 3 weeks ago and got this one)
ram:4gb corsair ddr2 800
cpu:6400 dual core
os:Ms xp 32 bit
psu:700watt

My problems right now are this, first when i try and install games it usually comes up with errors. I was installing warhammer online and everytime it would say I was missing a large amount of files took me about 9 restarts before it finally worked. Next I was installing diablo 2 and it would not work and got a corrupted install every time and I had to download it online. Next I tried to install rise of nations and everytime it failed to install because it could not extract.

After I manage to download or install the games the next problem is there are lots of problems with the graphics and i get wierd textures and colors. And sometimes I get the error the Nv4_display has stopped working restart your computer.

Nothing is over clocked, if anyone has any advice that would be great!!!

thanks

[gamer4156]

So I just replaced the 4gb of 800 ram, with my old 2gb of 667 and I can install the game fine. anyone know why this would cause an error?