Depends on what the resolution of your TV is. If it maxes at 720p then don't bother. Component cables come with the 360 and the max resolution they support is 720p. You'd need an HDMI cable for full 1080p, but most 360 games don't support this natively anyway.

If you do decide to get an HDMI cable out of laziness though (can't blame you, it's half the reason I did) and decide to go for something cheap, make sure it isn't super long. I'd assume you probably only need one a few feet long anyway, but some of the cheaper cables can have signal degradation the longer they get and you don't want that spoiling the HD image.