It's really up to you and your expectations. I suppose you could still buy digital and not get HD signals, but why bother at this stage. You would have to upgrade your tuners to recieve HD signals.
If 720p works for you, then you get a bigger screen for less. As the resolution goes up the screen gets smaller to stay in your budget.
720p good
1040i better
1040p best
Here's one that has good reviews and right at $700. It's also 1040p so you can be assured of getting the best picture possible with todays technology.
http://www.tigerdirect.com/applications/searchtools/item-Details.asp?EdpNo=4522367&sku=T24-4008