Right now I have two 512 sticks in my computer that are 400mhz. I am thinking of upgrading to two 1 GB sticks that run at 533. I know that the newer ones are compatable with my computer. My question is, can I leave the old 512 sticks in and just add the new ones, or do I need to remove them and only run the 2 1GB sticks?

you'll most likely have to switch them, unless there is extra slots. I didnt think you could put 533 sticks on a 400 bus but I'm electronically challenge. And waiting for someone elso to chime in because I have an older Celeron 566Mhz that I just all but killed because I installed XP on it and it only has 64 meg of ram.... WRONG MOVE!!!

id say if you have the room definitly leave them in, make sure you put the gig sticks in first (moving away from the processor) or youll get all bottled up.

I would say put them all in but they will probably only run at the slowest speed 400mhz...

they will, the system wont allow the memory to run at different speeds and if by somehow it did with like a cheapie generic mb then it would be insanely unstable.

i say get new memory cuz running two diffrent speed memory may work ok, but.....

it will not run the best, it will run slower then if you got matching pair of two 1GB sticks to add up toe 2GB like i did.

Newegg.com has good prices

The new ones will run at 400mhz, are you sure your mobo supports 533? because if the board itself doesnt support it, they will still only run at 533, most memory is backwards compatible speed-wise.

What are your other system specs? and what are you doing to notice a decrease in performance when you already have 1GB? In a "daily-driver" type machine thats typically more than enough.

it cannot run them DDR if oyu do because it will be trying to read it at two different speeds, i highly doubt your computer would even run depending on your MB

is it ddr or ddr2 ram cause there is a big difference.
Also, if you cant get faster speed get lower latencies. If it is ddr go for some 2-2-2-5 ram

I too am having memory issues on a IBM celeron, could you speak a little more in english cause i want to make the best choice for memory upgrade without taking it to a shop. 2-2-2-5?