No. No current card is bottle necked by any 16x slot.
As far as i understand it, your 16x slot is 16x 250 MB/sec both way data channels in PCI-E 1.1 - approximately 2x as fast as the last generation of AGP.
PCI 2.0 will double the rates, but there is no bottle neck to start with. The one thing your card needs the BW for anyway is for loading textures (Figure out how long it would take to load 512 MB of textures throught that data path? Not long! Even 1 MB of textures would be a blink of an eye!) which is typically done during the loading of a level in a game. In game, no need to worry.
The only time it might help you is if you are running bigger textures that those your card can support. The rest get cached to system RAM - so you are still going to slow down anyway on PCI-E 2.0.
-S
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