Currently the best thing I have bought for my 360 is a VGA cable so I can connect it upto a monitor. Now the funny thing is that I keep stealing my Dad's LCD screen from his computer, but his monitor is 14" and the portable TV that I used to play on is also 14". Yet everything still looks 100% better with the VGA cable.
Only problem I have had so far is that some games have strange aspect ratios in some of the higher resolutions on the cable, but like Gears of War a patch has been released to fix this and Gears looks especially nice on an LCD.
So if you have a monitor you can use, but don't have a HD-TV I would highly recommend spending £20 to get the VGA cable.