Have to agree with sminal on win 7 vs XP. The only holdback would be software or hardware. From the software there are a few that could be a problem and an upgrade can be expensive. Not had a problem with the software I'm using - One is a pain to install, but that is a software/hardware issue. Not a typical program, it's for transfering a signal captured on an O'scope via a serial port. The ONLY hardware problem was not a WIDOWS 7 fault) was the manuf trying to generate revenue by not publishing the driver (Canon Lide 35 scanner).
Although, starting with vista I was ticked off because of the inclussion of DRM - WHICH they Did not have to include and the system would have been better off without!!. I had no problems with vista and have converted all my computes at home to Win 7 (One dual boots to XP if needed. 3 Computer and two laptops.
On SSD, While they will definately improve, the 2nd generation - when compared to a HDD, at worth it. Desktop - Intel G2 80 gig. Laptop 1, 80 Gig Intel G1 (Not trim support), Laptop 2, 128 Gig Patriot torqx. Also have 128 gig WD blue, not sure where to use, may end up being my "scratch" disk for Desktop, or as spare data drive to share between 2 laptops. (one laptop hase esata, other does not - so bought a usb3 expresscard and a usb3 enclosure to house the SSD)
Bottom line, Upgrade to win 7 and I would recommend getting the SSD for operating system/Programs now, then later on get a "Newer" version and then relegate the first SSD to your scratch/working disk. The use the mechanical HDD for storage/backup.
An alternative to the SSD used as a scratch disk, would be to use a ramdisk as a temp work space while working on the file. But for this you need a large amount of phyical ram.