fglrx: OpenArena 40-60fps(fullhd,maxed out).
nvidia: With 9800gt - 300fps(fullhd,maxed out).
(for comparsion, 6800gt with gddr3 (asus v9999) on athlon xp 3200+ -> 120fps fullhd, maxed out)
Technically 4650 is weaker than 9800, additionally 4650 - 128bit DDR2; 9800 - 256bit GDDR3. So I think this is because gpu staves for the lack of ram bandwidth.
But then nvidia makes crap and closes drivers whilst ATI does A LOT for opensource drivers, so I bought hd4770(faster chip than 9800gt, 128bit GDDR5 on 4770==256bit GDDR3 on 9800; lower idle power consumption). I have it in my hware, Im trying to recompile lastest kernel to use it(typing this in vesa). Not much trouble, gentoo anyway.
The trouble is if this issues will be if all that options you talk (vid accel, 2d - is already here I know, improved opengl) will be handled in fglrx. Then it will be as in nvidia, but later and worser. I have switched to amd 4770 only because of their opensource effort, yes I appreciate it that much.