Hello Atelophobia, I think I can help somewhat, as I have quite a similar setup to yours, though it's been, erm, augmented a bit!
Firstly I have my PC setup with Vista 64, I haven't tried Windows 7 yet, I'm not entirely sure I will to be honest. I'm having (relatively) low framerates considering my PC as well: a QX9650 running at 3.6Ghz, memory at 1600-7-7-7 and a GTX 260 at 685/1455/1100 with the screen at 1800x1440, admittedly at max settings, but I'd still expect a higher framerate than 20-30 in the cockpit.
Anyhow, back to the controllers; I too have the MS FFB2. This works with no drivers at all in Vista, and for me at least is set up pretty much perfectly as it is. There's no auto centering in game other than the centering applied by BS, so trimming works well (or at least it will once I get my head around it!).
My x35t was a very old version, without USB support. so I gave it USB functionality using one of these: http://www.leobodnar.com/products/BU0836X/
Basically I ripped out all the wiring and electronics, replaced the pots and switches and wired them all up to the BU0836X. So really I can't help you with the x35t as mine works quite differently to yours.
I like the idea of fitting a different stick to the FFB2, I've been looking at picking up a second hand saitek stick for that purpose (the x35t came from ebay as just the throttle unit). The BU0836X might have been a little expensive considering a whole HOTAS unit might have been just as cheap in some places, but I had the FFB2 already, and using another ebay find and some ingenuity I've made a set of rudder pedals from an excercise stepper (!) and a potentiometer wired up to the BU0836X, so that's saved me a fair bit.