OK in reviewing my prior post I don't think I made it clear that the max memory is 1 gig, but the Max memory in one slot is 512.
NEWS FLASH:::::: THIS IS GIVING ME A HEAD ACHE. ok there are two similar models of your computer, one has a FSB OF 400 AND THE OTHER 533. DELLS WEB SITE SAYS THE MAX MEMORY FOR THE 400 FSB COMPUTER IS 1 GIG BUT RECOMMENDS EITHER 512 OR 1 GIG STICKS FOR UPGRADE. CRUCIALS MEMORY ADVISOR TOOL SATES THAT THE 400 FSB COMPUTER CAN TAKE A MAX OF 2 GIG OF MEMORY, I GIG MAX IN EACH SLOT. SO BEATS THE H*LL OUT OF ME. SINCE CRUCIAL GUARANTEES THAT IF YOU USE THEIR MEMORY ADVISOR TOOL THE MEMORY AND AMOUNT WILL WORK I WOULD GO WITH WHAT THEY SAY. THEY DO TEST SOME COMPUTERS AND FIND THAT CERTAIN COMPUTERS CAN EXCEED WHAT THE MANUFACTURS STATE THE COMPUTER CAN DO. SO HERE IS A LINK FOR YOUR COMPUTER. GOSH I HOPE THIS HELPS YOU OUT EVEN THOUGHT IT HAS BEEN QUITE A MESS.
Guaranteed-compatible memory upgrades for your Dell Dimension 4550 (400MHz FSB) Desktop/PC.
Have you looked at your BIOS yet to verify which FSB you have?! Second, if you can't make heads or tails of your BIOS, will you open up your case and get the motherboard manufacturer and model number?! It should be located between your white PCI slots in the middle of your board. Include everything it has inscripted on it. My guess is that you have the 400 MHz FSB. If a 533 MHz DDR board was put into a Dell system, it would have been put into an XPS system, as 533 MHz would have been a high end alternative in DDR systems. And you would most likely have noticed the difference when you went to pay for it when compared to the 400 MHz system. 
