[R] Speed up R

[Robert McFadden]

Dear R Users,
I hope that there is someone who has an experience with a problem that I
describe below and will help me. 
I must buy new desktop computer and I'm wondering which processor to choose
if my only aim is to speed up R. I would like to reduce a simulation time -
sometimes it takes days. I consider buying one of them (I'm working under
Win XP 32 bit):  
1. Intel Core2 Duo E6700 2.67 GHz
2. Dual-Core Intel Xeon processor 3070 - 2,66 GHz
3. AMD Athlon 64 X2 6000+
Or simple Pentium 4?

I'm very confused because I'm not sure whether R takes advantage dual-core
or not. If not, probably Athlon would be better, wouldn't be? 
I would appreciate any help.
Rob