I have been on crucail.com and had a look at memory upgrades. when its says select my system I put dell dimention. its says on my hard drive dimention 4550 but on there site there are two 4550
choices (400MHz FSB) or (533MHz Fsb) i dont know which one to choose.

Does either of these numbers appear on screen when you boot up? Most BIOSs (grammar nazis, is that right? ;)) will tell you the bus speed on boot-up. With a Dell you might have to press a key to see the BIOS messages though (and they will only appear for a few seconds).

If all else fails, you can probably get some sort of system diagnostic software that will tell you things - not sure about any free ones, have used Sisoft Sandra in the past.

One other thing that just occurred - not sure about Dell hardware, but if the two memory options are the same apart from the speed, see how much the price difference is. You should be able to just get the faster one and use it in either setup, often the prices are very similar.

There's a scanner tool on the Crucial website that you can download and run. It figures out what kind of memory you need if you're not sure. Always works for me in the office - we have some old machines where we're not sure of the specification. Run the scanner and it tells you.

Sounds a very useful tool, although it was unable to tell me anything other than this machine was made by Toshiba :)

ddr 3200/400mhz should be fine... its kind of industry standard.

Checking the specs for that PC, it looks like it's DDR 333, so DDR 400 would be good (it's usually a bit cheaper than 333 at the mo' too)