You are better going for a 1080p TV these days to be more future proof. The majority of 360 games are only 720p and ones that claim to do 1080p are just up scaled 720p.
However I would say get a 1080p TV if you can as it will be more future proof, if you ever get a Blueray player or pickup a cheap HD-DVD drive for the 360 your set for viewing HD movies. The next Xbox / Playstation will most likely have native 1080p games too.
It depends on how much you want to spend and the size of the TV, you can get a nice 25.5” 1080p TV for £380 on ebuyer - Samsung T260HD 25.5" LCD TV/Monitor Freeview 1920x1200 5ms Rose Black 3 Years Onsite Warranty - Ebuyer
Depending on the TV either Samsung or Panasonic were the best when we got our new TV at Christmas, to get the same offering from Sony you were paying a few £100 more for nothing more than the name.