Well I switch my 360 between my SDTV and my PC monitor (I prefer watching DVD's on the TV or playing some games while my PC is still on) and as one example, GTA4 definitely has a much smoother frame rate going for it when on the TV. As for 480p, well I can't make any comment but I imagine you would see quite an improvement on alot of visually hefty titles and it'll still look good too.
Alot of people underestimate 480p when it comes to image quality, but remember it's still outputting at a higher resolution than an SDTV and it does show.
EDIT: Grey Acumen I just re-read over your post and you say you don't own a HDTV, am I right in guessing that you're either going to get one in future or you're one of the few people out there with an EDTV?
I'd hate to come across as patronising but if you're not completely aware - Standard Definition TV and 480p are not the same as each other. Before the term ED (Enhanced Definition) came along, 480p was still considered a HD resolution and you're either going to need an ED or HDTV to display it.
