Windows and apps installed on the ssd will be fast. Everything else will be the speed of whatever data drive you put in, or the speed of the NAS you connect up to for data.
Most noticable speed improvements will be on boot up and opening any apps that are on the ssd. 40GB is going to be TIGHT though. I just installed Win7 on my ssd. Reduced the size of the swap file (since I have 8 GB of ram and probably wont need much swap), and removed hibernation (it also takes a huge chunk of space), installed basic utilities/apps, MS office, and MS updates, and I'm at 30gb+.
I was eyeing one of these for my HTPC comps actually. Seems like something like that may be the perfect use for one of these new SSDs.