SlickDeals.net has a decent Dell on their homepage right now, and by tweaking it you could get it in your price range (just take out the upgraded video card).
If you are going to buy a Dell, you NEED to use one of the high % off coupon codes or they're an extreme rip-off.
The biggest advantage to buying a Dell, in my opinion, is that they usually package in a nice monitor, so if you don't need one of these,
www.newegg.com will probably be the way you want to go as long as you're sure you can piece the system together with your friend.
Sony PCs are extremely overpriced, and everyone I know who has a Vaio has had problems with it, so from my personal experience I've got no reason to recommend them to anyone.
I've heard good things about buying through the Dell Outlet, however, you won't be able to customize the system and have a good chance of the 'good' PCs being taken by hoarders and resellers. Apparently there is a large following of the outlet center on FatWallet that knows the tricks to get nice systems for cheap; in other words, you've got a lot of competition.
I've had no experiences with iBuyPower and don't know anyone who has, so I can't give my opinion on them. From just screwing around building a system on their website, they seem to be a bit on the high end of pricing.
Depending on how much you want to spend ($600-1200 is a huge price range), what games you want to play, and what you already have for this system, there's a few suggestions on what sort of system you should put together.
Edit: Shit, if you don't need a monitor and are comfortable selling on eBay, I'd buy a Dell similar to the cheaper config on SlickDeals and eBay the 24" monitor, as it's fetching close to $800 right now. I'd expect that to drop *maybe* to $650-700 by the time you'd get your system, giving you a pretty good gaming rig (It's no AMD, but Intel has been working with some of the bigger games to get added speed for Intel Dual Cores via update patches) for $500-600.