Actually, with most people using Intel based computers, i haven't heard of mixing RAM as a problem (a guy i know for example has a 256 generic bargain buy from fry's as well as 512 corsair stick in his 2.0 machine). I dunno about Athlon systems, but i have rarely found mix-matched memory to be a problem for p4's, p3's or p2's (even with varying speeds - such as 2700 mixed with 2100 -- lol, now pc100 with rambus 1066 might be difficult lol).
I'm just your average habitual smiler =D