you're always better using to similiar styles of ram. Although since you're planning on buying another high quality ram I don't see any major issues. It's still not the preferable way to go, but as long as the new ram has the same timings you shouldn't see a major issue.
:cool: Save heating costs on your home, overclock your PC!!! :cool:
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).