If by multitasking you mean AIM, a web browser, and a game at the same time, than get a nice single core (3700+ san diego.) Dual core isn't gonna show you a big difference unless you're doing major multitasking stuff. However, you may want to go for an X2 3800+ dual core, because it will make this PC much more futureproof. But if you upgrade your computer every 2 years like I do, I would just go with a single core. However, you said "in 4 to 5 years" so I almost have to recomend a dual core.
DFI Lanarty Ultra-D
Opteron 165 @ 2.5, 1.325v
1x1GB Mushkin XP4000 @ 250, 3-4-3-8 (another gig on the way)
Sapphire X1900XT @ 700/800 (she\'ll do better, but I need to replace the stock cooler)
WD 1600JB 160GB
Antec SP 500W