Possible GPU Fan issue (?) with 2009 Mac Pro Dual 2.26

I've been using this machine since early 2009 and this "issue" has just seemed to appear suddenly about 2 weeks ago or so. The graphics card is the ATI Radeon HD 4870 512 MB.

What's happening is, when putting on a game (in Mtn Lion, Snow Leopard, or Vista) the fan will suddenly ramp up. Not gradually, but immediately. It's louder when there's actual gameplay and quieter during a menu for example but it's louder than normal either way (usually almost silent normally), but for example as more activity occurn onscreen it is immediately louder, but might slow down immediately when the action stops or the game ends. If something blows up (explosion) in the game you will hear the fans rev up loud for the second or 2 that it's on the screen, and then quiet down again, as one example.

With normal use it is almost back to its quiet state but seemingly a tad more audible than I can remember it being, but not much. Mistat, and istat Pro show Boost A and B (I Don't know what those are) about 1800, give or take, during normal use (it's at about 1700 as I type this) and hits roughly 2200rpm or so during a game. The Expansion is usually at about 900trpm or a little over 1000 but i don't remember them before so don;t know if this is alarming or not. All Temp readings seem to be normal or average from what I've remebered or have looked up elsewhere. GPU-Z monitor in windows states the GPU temp at about 90C, give or take and that seems to be about normal according to what i can find on it.

Hardware test turns up nothing and Applecare has (naturally) expired a while ago (without the option to renew). SMC was reset twice. Computer is cleaned out for dust inside every 6 mos. A replacement card seemes to be gone from the store and the price for the other compatable ATI 5870 is still (a ridiculous) 400+, so that's a last resort for now. I just wanted to try to find out if this was a sign of a current or looming problem, or just possibly normal effects of "aging" equipment and thus nothing to be concerned about considering the circumstances. Any input welcomed, of course.

Thank you.

T