I'm trying to play some Steam source stuff (Half Life 2: Lost Coast, Half Life 2: Deathmatch, Team Fortress 2) on Ubuntu 12.04 LTS, but the performance sucks. I'm using a Sapphire Radeon 2400 XT 256Mb GDDR3, with 4Gb RAM, and a AMD Athlon 64 X2. On Windows 7 x64 with maxed settings and 4AA/16AF I get about 20-40fps. On Ubuntu 12.04 LTS 32bit I get about 5-10 with 0AA/0AF.
Is there anyway to improve the performance? Even lowering the settings to minimum doesn't change the fps much. I've also tried the xorg-edgers PPA but that didn't improve the perfomance at all.

I also know that I can install the fglrx driver but I've had nothing but problems with that before, so I would much rather use the open drivers.