If I remember correctly, I've seen an article not too long ago showing 8GB to be slower than 4GB in most games. So if gaming you might want to stick with 4GB.
Umm, maybe they tested 64bits version against 32 bits version of some games? Like Crysis 32 bits used to run faster than Crysis 64 bits ( at least when it was out).
edit: Where you talking about that article? http://www.bit-tech.net/hardware/2008/07/08/is-more-memory-better/5
I looked at it pretty fast but it seems that sometimes its slower, but usually a tad faster. One thing for sure though, 8GB makes vista happier XD