Laptops are very limited in upgrading. You can usually only upgrade the hard drive, RAM (but usually not a huge upgrade), and the processor (again usually limited to a processor with very similar specs).
Your dell can only support 2 gig RAM max and it looks like it uses a pentium M processor that can be upgraded at most to 1.7 ghz with a 2 mb cache.
As for the hard drive it says that they come standard with 30 gig, so you could upgrade that to almost any size you want assuming you get IDE / ATA.
Found here http://www.dell.com/downloads/us/pro.../d505_spec.pdf