In reply to ChrisJD:
Increasing travel will make forks LESS stiff, not more stiff.
Tapered headtubes increase stiffness in quite an obvious way - the contact point from which the fork flexes is 40% bigger. Snap a pencil vs snap a thick bamboo cane?
A 15qr lower will help, and if you are unwilling to do anything else is likely the only reason to change your forks.
You could search ebay or a tuning company eg J-TECH and see if you can find some 15mm Reba lowers, or just buy a pair of Reba's off ebay with f*cked stanchions / broken internals and strip them yourself its easy.
If you can get a 120mm fox 34 (I don't think so)? You may be able to find a 1/1/8 lyrik that's got a 15qr that you can spacer down to about 120 - but you will be paying for it by increasing weight quite a lot.
Are you sure this 'flex' is actually affecting your riding that negatively?
There are a lot better ways to spend a couple of hundred quid - a reverb, petrol money, plane tickets, a new frame, nicer clothes to ride in etc etc