mmm, friend, unless you are going to get a whole new system, PC3500 or PC3700 RAM can only decorate your bookshelf...
If you ARE going to get a new rig, I would say by the time PC3700 should be widely available and many potential problem will be fixed. A good example is some early batch of Corsair XMS PC3200 has serious problem when the frequency and timing is set to [By SPD] or [Auto] on nForce2 mobo, but later the bug fixed. Now only Corsair offers the PC3700 IMHO even it is good quality but I won't recommend anything without competitor.
BTW, I can tell those Kingston HyperX PC3500 blue monster is screaming. I have two of those running in my rig now, rock stable, and oc extremely well; from review I've read, the HyperX PC3500 is head to head with Corsair PC3500 TwinX. Corsair has higher overclockability if run at loose timing, but when timing is lower, Kingston rulez; but from the roundup (Corsair vs Kingston vs Mushkin vs Geil), Mushkin PC3500 is the overall winner while Geil goes under bottom of the sink...
It is morally wrong to allow suckers to keep their money
<A HREF="http://www.anandtech.com/mysystemrig.html?id=22996" target="_new">My System Rig</A>