Off-hand, I don't know the limitations of the motherboard in your Dell so I would check with them to see if what you can go to for sure.

For the RAM, yes, you can put faster RAM in there, but it's going to be worthless as you won't be able to make use of the higher speed options past PC3200. PC3200 corresponds perfectly with an 800MHz FSB P4, and since you really can't do much in the overclocking department with the motherboard you have, it doesn't make any sense to buy PC3500, for example. You can still buy, say, PC4000, but the additional speed ceiling won't do you any good. You can increase the capacity, however.