Your Dell XPS 420 probably has 4GB of DDR2 667MHz RAM. You could upgrade it to 6GB, if you have use similar DDR2, 667MHz PC2-5400 memory modules, but normally Dell's BIOSes are locked, and you cannot change the memory speed, so most RAM would just run at 667MHz.
Just so you know, RAM at 1066MHz is either PC2-8500 DDR2 or PC3-8500 DDR3. RAM running at 1600MHz is definitely DDR3 PC3-12800, and your motherboard would most very likely only be able to support DDR2 RAM.
Hi newcomer and welcome to the Tom's hardware forum.
1- You can upgrade your RAM for 1066, but is better that you go for 4GB 800MHz, because your mobo don't support 1066. So, if you buy 1066MHz will be a waste of money. And it's not true that the BIOS dell is locked.
2- 1600MHz is DDR3, RAM that your mobo don't support. Your mobo only support DDR2, and the max speed that you can find in DDR2 is 1200MHz, speed that your mobo don't support too.
3- Finally, Your mobo only support 4GB max, so, you can't upgrade to 6GB
I did a lot of research myself and came away completely mad.
You see...I have the Dell XPS 420 Quad Core (2.4GHz) and a Dell XPS720 Duo Core (2.67GHz). Both systems have the same RAM.
The power supply in the 420 (350W) is too small for the nVidia 9800GTX+ I have in the 720, and the 720's dual core is too slow for my needs. To top it off, the 750W power supply in the 720 won't fit in the 420 and the maximum the 420 can take is 425W.