I don't know if you have used the dell support forum, but if you haven't you will see that they have a number of threads on tis subject. Like you, I'm unsure about the upgrade options for a dell.
I think it comes to:
1) new chip (I can go to only about 1.1ghz (I don't know what your limit is)(Cost £50-100?)
2) adaptor (with powerleap I could goto 1.4 (cost of at least £120 in UK)
3) new box, psu, mobo and cpu and then transfer hard drive, CD-RW,DVD into new base system (perhaps abit more than the powerleap but with a more powerful spec)
4.totally new system
5. check out flecc's Strewth thread about getting more spped with the existing hardware.
I don't know if this helps.