Oooo, lovely price range
Are you overclocking?
Hopefully you can wait a little while until the new GPU's come out, and reach the distributers. In the mean time, I put a 6800gt in there.
It still needs a case, but you should choose that, just make sure it is ATX.
I didn't put a monitor on as well, as I wasn't sure if you wanted it.
As you said you wanted multitasking, I put the new dual core AMD 64 on there (the 4800
) If you want, you could put a FX 55 in there, or a 3700 san diego. If you do alot of multitasking (e.g. decoding a video, graphics, and playing a game at the same time, than a intel may be better.
I put 2 HDD to be put in RAID 0 for improved speed.
DFI LanParty UT NF4 SLI-D (Socket 939) PCI-Express Motherboard (MB-013-DF)
OCZ 1GB (2 x 512MB) PC3200 Dual Channel Platinum Series EL-DDR REVISION 2 CAS2 (MY-048-OC)
Seagate Barracuda 7200.7 Plus NCQ 160GB ST3160827AS SATA 8MB Cache - OEM (HD-015-SE)
BFG GeForce 6800 GT OC 256MB DDR3 TV-Out/Dual DVI (PCI-Express) - Retail (BFGR68256GTOCX) (GX-000-BG)
AMD Athlon 64 X2 Dual Core 4800+ (Socket 939) - Retail (ADA4800CDBOX) (CP-129-AM)
Enermax Noisetaker 600W EG701AX-VE(W) SFMA ATX2.0 SLI Compliant PSU (CA-021-EN)
Creative Sound Blaster Live! 24-bit 7.1 PCI Sound Card - Retail (SC-029-CL)
Zalman CNPS7700-CU Ultra-Quiet CPU Cooler - Retail (HS-010-ZA)