no beeps, no screen, no post?

psu: 420w hpc-420-102 df
processor: intel p4 3.12 ht
bios: intel bz87510a.86a.0036.p07.0306170845
hd: 200gb wd2000jb-34eva0
ram: 2gb ddr (4x 512mb)
os: winxp 5.1
gfx: ati radeon 9800 pro 128m

this computer has been running fine for 2 years as a gaming pc. a few days ago, upon startup, it beeped displaying a bios message stating that the ram had been decreased to 1gb. i reset the cmos + ram, and the problem was fixed.

since then, the graphics started getting messy during gameplay; i.e.: shadows first became blue rather than shades of gray & eventually would turn vibrant colors such as canary yellow. this could be fixed by simply restarting the pc.

this morning i tried to startup and during post it beeped 3x, then booted fine. i loaded my game of choice (battlefield 2) and noticed that the intro was lagging; sound & video. i dumped all cache and temp folders and restarted the machine, but the screen never came back.

i reset the cmos + ram again, and got nothing. i figured, in lieu of the events pertaining to the messy graphics, that my graphics card had gone bad. i bought a new one and installed it with no change.

i removed the graphics card all together to see if i would get the beep codes during post, but i didn't. the odd thing about the whole matter is that all the drives cd/hd spin, and the "thinking" indicator light comes on momentarily.

my optical mouse never lights up, as it usually does during post. it seems as though it's being halted during post somehow?

i tried using only 2 of my 4 ram chips & i've checked all the cables to make sure they're all connected. as far as i can tell i haven't any shorting problems. i am completely stumped. any suggestions?