I don't know about the ps3 on an sdtv, but the 360 looks fine. HD just makes it look sharper, glossy, brighter, and prettier... But seriously, playing it on an sdtv does NOT lower the polygon count or the fps.
jesterslave, you might want to check out your local electronics store. Its easier to compare what your PS3 will look like on standard tvs vs HDTV's there. I think the Samsungs and Sonys are the best on the market.
Ive been told 480p is standard resolution and 1080p is HDTV resolution. Is 720p HDTV as well?