hooking up pc to HDTV

okay so ive moved my PC into the lounge while my flatmates are away to use the 32" for gaming. Howver I cant get a 1920x1080 res when I hook it up using the VGA cable i'll only give me a max of 1360x768. but then when I use a VGA to HDMI I can get full res (but it looks terrible). The thing is, I'm pretty sure I've run it at 1920x1080 on this tv before. anyone know where I'm going wrong this time.

fyi

tv: Samsung LE32a457c1d

card: ATI radeon HD4870

OS: windows 7

If your TV is 1080p, its best to go DVI -> HDMI. VGA to HDMI isn't doing anything but replacing the connector. The TV doesn't seem to like VGA inputs of high resolutions (oddly enough a lot of TVs dont for some reason.)

thanks guys