I wouldn't worry about the performance... there are YouTube videos of the 320 40GB booting up Windows and it looks pretty snappy.
In fact, going from a standard mechanical HD to an SSD you'll notice a big difference, no matter what the SSD, even the most value oriented ones.
So I say go for it... although while performance isn't a worry, the size is perhaps a little tight, you won't be able to put much on 40GB after the OS, obviously.
You could maybe consider something like that
Corsair Force range which gets a good review there as excellent value for money considering the performance, of course that's the 120GB model but you could go for the 60GB one.
Either that or I've heard good things about the Crucial M4 64GB which might not be the best rated in terms of tech specs, but it has very good real world performance, and again isn't going to break the bank.