I would put it more likely in the department of how much ram you would have, or how many processes are running around in the background....
I have one of my Terabyte servers running on a Pentium3 800mhz cpu, but it has 1 gig of ram... I regularly stream 2 movies at a time from that server, to 2 out of 3 of my Modded Xbox's.. hooked up to 2 different plasma's.
So thoughput is not an issue at all... nor processing speed, as that thing is a slower then dirt processor, but it does its job just fine, running winxp.
Also check what processes are going on in the background. I usually totally wipe my system after 1 year or sometimes less, to clean that eroneous junk that builds up over time...
I make an image of my OS hard drive, and it literally reloads windows within 10 mins, back to brand spankin new...
Warp