Yes, I would very much advise sticking to the same speed. You will want to look for DDR2-533 RAM. By the looks of the CPU-z info, it is just standard DDR2 RAM so it should be an easy upgrade unless Dell has done something really funky.
Ok, heres the scoop. My B-day is today so my girlfriend got me some more ram (witch is y i was asking questions first). Now i have the ram put it in slot 3 (cause i have 2x 512), boot up and as soon as i power up it starts beeping like crazy and my fan (witch is normaly cpu controled and very quite) winds up to max rpms, and nothing displays on screen. So i shut it off and put it in slot 4, boot up and its still not acting normal but is not spazing and i get on the screen:: R423SE P/N 113-A32102-103-8P A12 BR#7891 Rel 9.3.17 :: At this point im thinking it might be the chip and not the install so i remove all sticks and put the new on in slot 1 boot up and it freaks out again. The recipt says the stick is a : OEM 512MB DDR2 PC2-4200 8C 533Mhz 240 PIN DIMM : witch is what i need, however the sticker on the stick says its a PC2 4300. Could that be where my problem is or what. Also i told her to get 2 sticks and only one came (witch im working on that atm), could it not be working cause i am to use 3 sticks and not an even number? I am greatful of any advice.