Having a dedicated machine just to stream stuff to your PS3 to TV sounds overkill to me.
I have a similar setup, but I just run PS3 Mediaserver on my desktop machine. It starts on system start up and I never notice any overhead, etc...
However, if you do wanna build a seperate box, don't limit yourself. Soon you'll realize that if you already have a dedicated machine, just skip the PS3 all together and use the PC directly.
Might be some extra cost, but a nicely built proper media pc will destory a PS3 serving the same purpose - especially if you can have both
But im poor, so rather then blowing cash on a dedicated mediapc, i just use PS3 media server on my desktop, streaming to the PS3.