That card is fine. OS bit is not directly related to the card bit. Card bit is the memory architecture of the card. The only thing that will effect the card as far as OS goes is if there is no 64-bit driver available for it due to it’s age. That card has the 64-bit driver and the 32-bit driver available, so it’s good. So basically, it depends on age of card and if the manufacturer made a driver for it for whatever OS you have.
Bottleneck as in overall performance? Or video performance? Video performance you could say RAM as Windows 7 and Vista I believe (not sure about XP), share RAM with the video card depending on the application.
Overall performance, could be a million things but my bet is usually on the OS needing a good cleaning (temp files, start up apps, etc.).
:lol