honestly I don't think you'll notice much of a difference between that hard drive and a 7200rpm one
this hard drive has been known to have excellent performance, not to mention it's much bigger:
Newegg.com - Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives
I'd probably go with this one though, since you seem to think that 74GB HDD is enough... this hard drive is cheaper and has a 32MB cache:
Newegg.com - Seagate Barracuda 7200.11 ST3500320AS 500GB 7200 RPM 32MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives
q6600 is fine if you're gonna multitask a lot... but i mean a lot.
the E8400 would really be fine, and it's gonna outperform the Q6600 in many areas, unless you overclock the Q6600
Here's some better RAM - at least in my opinion (i like g.skill)
Newegg.com - A-DATA 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Desktop Memory
or drop an extra 5 bucks and get slightly better:
Newegg.com - G.SKILL 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Desktop Memory
What case and power supply were you thinking of? You posted the video card twice.
I don't know too much about that motherboard... don't think it's too popular.
oh and also get an aftermarket cooler for the cpu, especially if you go with the q6600 - xigmatek rifle is good, but a pain in the *** to install.
Are you using XP or Vista and do you already have the OS? If you're gonna run vista, imo it's worth it to get 4gb of ram, xp is fine with 2.