If I recall correctly, OnLive needed one server for each client, in contrast to Gaikai. Now the funny thing is if you look at the costs per server, figuring they could handle two ps3-era games at most, it'd be cheaper to just produce a special rackmount PS3 with network storage for everything (no blu-ray, no HDD). You'd end up with one user on a $100 server vs. two users on a $600+ server.
The other problem with using PCs for streaming PS3 games is you still have to figure out how to run the games on them. If standard PCs could run PS3 games easily, we'd have BC in the PS4 and all this would be academic.
Of course, the problem with using PS3s is that it's harder to integrate the video encoding software, so they'd either need dongles that plug into the HDMI ports, a special chip on the board, or possibly just enough extra memory so they can run a fancier remote play (8 spu Cell maybe?). And they'll also lose the advantage of reducing lag by running at a high frame rate.