Things that you may do on a single core now:
Athlon64:
1. Games
2. MP3 Encoding
Pentium (HT's, not the simple single core's only)
1. Video Encoding
2. Multi-tasking (Any desktop related work)
-> Also such single core solutions are if you don't have the bucks for the dual-core.
Dual-Core:
AthlonX2 and Intel's Conroe (or if you can wait Merom, not the
:
1. Games
2. MP3 Encoding
3. Video Encoding
4. Multi-tasking
-> Though i would suggest AMD's, if you are thinking of upgrading later. Don't get me wrong, Intel Conroe and Merom will be good, though what i don't like is that they change Motherboard designs every around 6-12 months. So the new proc that will come out later usually won't be available for the mobo that you bought.
NOTE: I am though an intel boy.

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