The question is moot if you are running 32-bit XP or 32-bit anything...4GB is the useable.
I cannot imagine doing anything that would require 8GB of RAM in 2010, on the systems that I have. I only do mundane things that any computer made in the last 3 years (except maybe netbooks) can handle easily with 4GB of RAM.
What I'm asking (indirectly) is...what do you intend to do that would benefit from 8GB of RAM?
As for speed...depends just a tad on the motherboard and what it likes, IMO. My boards would favor the 4GB of PC2800.
The ultimate answer will probably be...personal preference, but I cannot see how having more of slower RAM could be beneficial...when talking about 4GB compared to 8GB. If you were comparing 2GB to 4GB, I think I'd go the other way...4GB is (IMO) significantly better than 2GB.
Now...I've confused myself