I would go with DDR2-800 instead, not a noticeable drop in performanceCorsair 4gb DDR2-800
$55 ($30 after MIR)
The WD Black 640gb is $5 more, gets a little extra performanceWD Black 640gb
I'd spend a bit more on the gfx card, at least a GTS 250XFX GTS 250 512mb
$130, free cod5
Also, please indicate your budget for the build and intended use for the pc.