More important, does your system support 64bit? If your processor doesn't support 64bit, it won't even install. Also, make sure that all your hard and software runs on 64bit (support isn't nearly as good as 32bit yet). 64bit is really only interesting if you have over 3GB of RAM. Otherwise, you may see a slight performance increase in some situations, or a small decrease in performance in others (longer strings may put more stress on the bottlenecks like FSB).
To be honest, my current system has 2GB of RAM (looking at adding 1 more GB in the year to come), an E6600 and Geforce 8800GTS. I have decided to get the 32bit OEM of Vista Ultimate, for compatibility reasons (have tested 64bit Beta2/RC1/RC2 on this and my old rig), after running XP64 for 2 years (giving me a lot of support/driver headaches). If in the next 18 months 64bit computing becomes more mature, I may switch and pick up a 64bit OEM as well (taking a chance MS will activate this 32bit again on my old AthlonXP), but for the moment in my situation 64bit has caused me more trouble than benefits...