Thats what I have been upto too ... Upgraded motherboard to Asus Stricker II Extreme, RAM to 8GB 1600mhz, Nvidia GTX550 Ti 1GB graphics card and Windows 7 64bit. What have a gained ...well a faster PC and improved performance on high end games BUT a number of other games now don't work no matter what compatibility mode I use
SO to get round this I have created a virtual machine using VMPlayer to emulate the old one
I did think about going down the SSD route as I have a small 32gb drive but the only real advantage is load times but the cost for a decent size SSD is just not worth it YET
Tunning your car cost to much these days (and you get dirty) so TUNE dem PC's its clean, its dry, its warm and fraught with just as many disasters to keep you pulling your hair out
what more could you want...