yes, you can mix makes (but they do not always play well together, sometimes won't work at all).
as far as the DDR400 goes, almost all mobo's will not accept any more than 2 DDR400 chips, and the ones that do, will reduce the speed below 400MHz to keep the system stable, but this will create a bottleneck in the system and actually give you worse performance than the 2 chips alone!
as far as a third memory chip goes... why bother with another strip anyways? i'm willing to bet that your system barely (if even at all) uses the 2nd chip, nevermind extending onto a 3rd!
the reason for this is that the mobo (more specifically the SIS chipset) you are using does not support dual channel memory, and unless you are running a dual channel setup, the 2nd memory stick isn't used until the 1rst stick is full, in dual channel -both chips are used at the same time,that is where the real performace lies my friend, dual channel doubles your bandwidth from 3.2GB/s to 6.4GB/s!!
don't worry about the AGP slot, AGP is old technology, all socket A motherboards have it (if you could find a board without one, it would not be able to support your CPU anyways)
-------->conclusion, your best bet is to return that 3rd chip and buy a mobo with the NForce2 Ultra Chipset, running 2 512MB chips in dual channel mode will be more than enough (otherwise get 2 1GB chips, just make sure they're a matched pair)
hope that helps..