As the two gents above mentioned, the legal limit at fields is 300 feet/sec. which is for safety, but also is about the ideal speed for accuracy as well- most manufacturers quote the ideal speed for their guns to be in the 280-300 fps range.
HOWEVER, I believe you asked how fast CAN a paintball go, not what the field limit is. SO- if you're not playing a game and are just messing around, you can crank your gun up, but I don't recommend it.
A few times at my old field, a few of us were doing just that on the range, and we got a few guns consistently chronographed at over 380-90 fps with regular paintballs, but we also got a few barrel breaks and poor accuracy.
One ref brought out a few frozen balls (he would always threaten cheaters with them!), and we were were able to top out at about 425 fps. Under perfect circumstances, with the right gun, barrel, and thick skinned or frozen ball, I imagine 475-525 fps would be attainable, maybe more if you did some customization al la "Mythbusters."
That said, 300 fps isn't just a good idea- it's the LAW!!!
18+ years of paintball playing
2 years of working at a field