A new mobo would mean a new copy of Windows. If you were going to go that far, just build and start fresh.
Of course, that would sort of push your current computer off to the side and all.
With the upgrades I linked to, it would run faster overall. Faster CPU, more RAM, much better graphics processing ability. If you build, you can utilize the newest CPUs, like a quad core AMD (to keep costs down). The Athlon II X4 line is very nice with great performance and still easy on the wallet; the 620 runs around $99.
Give us your best budget and we can see what we can come up with.