I guess what this comes down to is really a recommendation. Here's mine.
For the vast majority of people out there, a single fast processor is better than two medium speed processors. Compare that Athlon MP at $140 to an Athlon XP-2800+ for $100 or less. Realize that a lot of applications cannot use dual processors, and won't run any faster. Also look at the cost of dual-CPU Athlon XP mainboards. They're hard to find and really expensive, whereas a top-end single CPU Athlon XP mainboard is about $100 or so.
If I was going to spend the kind of cash it took to build a 2-CPU Athlon XP mainboard, I'd go and buy an Athlon 64 CPU and mainboard; for the same price it's far more advanced technology and performance should be equal or better in most cases. If I wanted to buy a budget system, I'd either buy an Athlon XP 2800+ and a single CPU mainboard, or I'd buy a mobile Athlon XP-M 2500+ if I was an experienced user and overclock the crap out of it, thereby getting Athlon XP 3200+ performance or higher.
I'd skip the Athlon MP processor.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890