SSD for XPS 700, How to select

Hi I have an XPS/Dimension 700 which I'd like to upgrade with an SSD disk.

How do I select the right one ? 

What can I expect ?

Any special problems challenges to look out for ?

I can see from my systemconfig that I do have a 250GB (2X250GB) 7200RPM SATA HARD DRIVE

My CPU is XPS700 CORE 2 DUO PROCESSOR E6400 (2.13G

How much work is involved if I like to copy my existing Vista Ultimate to the new drive, or do I have to start from scratch and upgrade my OS to W7 at the same time ?

regards

Martin