The only reason to get Dell or HP is price, meaning you get the cheapest you can find for under $500, else check around the sites, there're plenty of builders that would love to have your business.
For $1200, you can get a decent gaming rig with Athlon 64 3200 (best bang for the buck now IMO) with 512 MB RAM from
Cyberpower,
Digital Storm,
iBuypower,
Tantus, and
Xtreme Gear.
You can get many sweet cases like X-Blade (my current fav at Cyberpower), Thermaltake XaserIII, Raidmax Scorpio 886, Chieftec Dragon to show off with cold cathode light.