Ok, this may be a question that most of you know off the top of your head. Anyway, Sony Vaio Computer Pentium 4 CPU 3.20GHz 3.19 GHz 512 MB of RAM. Can I add a New Ultra 512MB PC3200 DDR 400MHz CL3 Memory ULT30215 to my system???
if you have extra dimm slots(its where the ram sits) on your board then yes, if your board is compatable with that ram. best bet is to have a look inside the case and buy the same brand that you have already got. that is assuming your pc runs on ddr, and not ddr2.
most likely you have an available ram slot, so in short, yes. However more importantly read your owners manual and find out specifically what type and speed of ram it uses and get that same kind. If you get a ram speed higher than your mobo supports, it won't work, and if you get one slower that what is already in there, it will make both sticks run at the slower speed.