its because your 6800 has 128MB of video RAM. the texture data size, AND the 2xAA/4xAF just fills up too much VRAM for 128MB. thats why your framerates drop. I suggest you turn off AA and AF and increasing your resolution. check out this, it shows a 6800 on a Athlon XP 3200+ and a 6800GT the numbers should be about right:
if that doesn't fix it, consider a prossesor upgrade. but it doesn't look like you need that
on my system, i run HL2 maxed out NO AA/AF at around 20-60fps. and thats on a 9500Pro!