You can run 32-bit software. Athlon 64's can run both 32 and 64-bit software.
Even if you have a 64-bit operating system, you can still run 32 bit software. Just not 16-bit legacy DOS programs.
If everything (software and hardware) are optimized for 64-bit, it will be faster. Get loads of memory too. Especially in intensive programs where lots of calculations are used, you will see a big increase in speed.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?
Yuck... the pent. d = yuck. The sempron is old... the athalon 64 3400+ = yes, get that. Make sure it's socket 939 to0, so you can upgrade to dual core's later on. Or just an X2 4200+ and save yourself the trouble.
Macbook Pro and Logitech z5500s. All you really need.
Socket 754 doesn't support as high as bandwidth either....
Anyway, @ $Thread_Title :
Yes, the 3500+ is a good solid CPU thats at 2.2GHz and can be OCed although I wouldn't recommend it with stock cooling, if you get arctic silver 5 thermal paste and a nice zalman cooler...you could OC it up!