I'm quite baffled as to which version I should buy for my laptop. I run a...

Dell Inspiron 6000 running Windows XP Home
1 GB DDR Ram
1.4GHz Intel Celeron M Processor
128MB Intel Graphics Media Accelerator Driver For Mobile GFX
60GB HDD
Sony DVD-ROM drive

I think that should be enough information. I am looking to purchase a nice sized mobile hard drive for my laptop, I'm thinking 320 gigs should be good, or 160.

My question is which version of Vista is right for me? I don't know if I can upgrade the RAM or not (it runs just fine for me with 1 gig).