Good points, tomos!
In ye olden days, you'd have nothing but trouble if you tried to mix and match memory modules, especially when dual-channel first arrived. Heck, even something as relatively recent as the nForce4 chipset tended to have problems if you loaded all four memory sockets with identical
memory modules, and would go from DDR-333 to DDR-266 by default (and that's even though the AMD64 memory controllers are on the CPU, not the chipset...)
The situation is supposed to have improved a lot by now, but I'd definitely not use different-speed memory modules (because you're limited to the slowest speed used), I'd prefer modules with the same latency timings, and I'd do my best to find identical
modules. And you can't necessarily expect to be able to find identical modules if you wait a long time before upgrading - especially if you buy "old" memory modules (ie, DDR in the time of DDR2, DDR2 now that DDR3 has hit the market (with insane prices)).
I went all-out and got 8 gigs for this machine. It's long and far between I use more than ~4 gigs, even though the extra filesystem cache is nice enough. But I reckoned that DDR2-800 memory was cheap enough to go for it, and I wouldn't have trouble finding matching modules later on