Between 480 to 720, yes. If you were to compare a 720p component signal versus a 720p HDMI signal, then most likely no, or very little difference.
Go here and buy your cables, you'll thank me later.
www.monoprice.com.
Here's the one I bought:
6' HDMI cable
Don't ever pay $100+ for a Monster Cable, or similar name brand. I got my 6' cables from here, and paid $20.00 shipped. I had a set of Monsters I got from Best Buy, and compared the two when I got the monoprice one, and the monoprice one had far better construction. Besides, the signal is digital, so as long as it gets from one side to the other, the is no signal loss.
Here's a good forum to read up on:
www.avsforum.com