Basically, this computer will only be used for gaming. Goal is to get a very high fps rate at decent graphics setting for those high-graphic games (CS:S makes hurts my current computer

Currently have a Radeon 9800 graphics card, but I think my current computer just wasn't good enough to get much out of it (~30 fps on medium low graphics settings for most games, also very choppy). Think it needs to be upgraded?
As far as going the custom route, my knowledge is pretty limited, so building my own is very intimidating. For this reason, an off-the-shelf system would be preferred.
Budget: Would prefer to keep the cost around a grand, but willing to go up to ~$1500 if needed.
Thanks to all for any opinions.