This is a good buy
http://www.perth-wrx.com/vb/sale/38446-dccd-6-speed-gearbox.html
Make sure to put a plated rear LSD in if you go for a DCCD box. Its also a damn good idea even if you go non DCCD.
Anything less will make baby jebus cry.
Otherwise if you just want to fix all your notchy gear shifting.
I would recommend:
Group N transmission mount and engine mounts.
Kartboy shifter bushes in the uni joint, front bush and linkage rear mount.
And change the oil. The Motul oil like Joe said is good shit.
My preference is still the Redline lightweight shockproof. But you MUST drain your gearbox thoroughly. I'd let it drain at least over night. If your car is not a daily driver... let it drain for a fugging week. Redline does not like mixing with the remnants of the old oil.