You can go to Dells website and enter the service tag for your PC and it will tell you what upgrades are available for you PC.
You will need to figure out what speed you need and go from there. IIRC mine is DDR 2 5300 . My Dell is a Dimension 8400 built in 2004 is running XP , so I have 4 slots . I upgraded the RAM in my Dell from 1g to 4g. Its very simple. IIRC its about a 1 beer job, but if you like it can be stretched to a 2 beer job....
Once you determine what RAM you need , might i suggest a wrist grounding strap. they are about $5 . It keeps you grounded while you're working inside the chassis of your PC. Static is very bad.
Remove all CD/DVD 's from the trays and everything from the rear of the tower.
Once you open the tower, ground yourself, Gently pull the RAM from the mother board and replace it with your new RAM, just make sure its positioned in the slot and firmly seated.
While you in there you can use some compressed air to clean the dust from everything preferably before you start. Then close it up, reconnect everything and Fire it up.