What's your budget? That would be nice to know....
I don't think you'll really need a GB of memory or PC3500, for that matter. An Athlon XP 2600 probably won't take much advantage of it.
That's pretty damn good. CAS 2, PC3200, dual channel, 512 MB. I would strongly recommend getting that. Even if you upgraded to an Athlon XP 3200, that PC3200 is probably your best bet.