I have exactly the same problem. Never had any problems with XP, but now after installing Windows 7 Pro, 32 bit, I get the very same out of memory errors, I can fly around with 40-45 frames, and suddenly BAM - out of memory.
Running in compatibility mode makes no difference.
I have the following specs.
Motherboard: ASUS Maximus Extreme,
RAM: OCZ Platinum Dual Channel 2x2gb,
CPU: Intel Q9650 Quad core 3.0 Ghz,
GPU: MSI GTX280-1GB Ram Super OC,
Power supply: NorthQ 1000W,
CPU cooler: Asus Silent Knight II,
Case: Raidmax Katana / Miditower,
HD: WD Caviar Green WD10 1TB,
Monitor(s) 22" flat + 19" flat, Windows 7 32 bit,
Flight Simulator X, Acceleration!,
Saitek X-52 HOTAS flight stick set,
Saitek Pro Yoke, and Throttle quadrant,
Saitek Pro Pedals, Saitek Pro Switch Panel,
CH Yoke,