first off all the ram in a computer MUST run at the same speed so the new ddr400 would have to run at the old ram speeds for them to be both used
i suggest getting 2x512MB sticks so they can run dual channel
i have a pair just like these expect they are DDR2 800 5-5-18 right now i have them running at DDR2 866 4-4-4-12
i can also run them at
DDR2 1080 5-5-18.
A-Data the best price per performance out there. there better than corsair XMS2 overclock the same but cheaper
im sure the DDR400 series of A-Data is just as good