For more discussion, I suggest re-opening your thread in the hardware forum but since it's here now, here goes.
"MX" cards are crippled, low performance versions and I would not recommend one. FWIW, I consider under 45 fps "low" in flightsims based on around 20 years of gaming experience. Flightsims seem far more smooth and realistic to me above 45 fps. I suggest that you aim for above that.
If you don't have much money, try finding an nvidia GeForce4 4200 or 4600 for a bit over $100. After that, you could try an ATI 9600XT or an Nvidia FX5900 XT for between $150 and $180ish. Above that, you spend whatever you want up to and beyond $400 with the Nvidia 5950 cards and ATI 9800 XT cards.
Currently the high end ATI cards seem to have an advantage in most games and in image quality, but before HiTech started using an ATI card, AH2 was definately better on nvidia cards. I suppose AH2 will probably work equally well on nvidia and ATI cards when HiTech is done optimizing for both types of video card.
I personally use a GeForce4 4200 and it runs most games just fine, but it's definately not as good as the newest cards out there. I can average over 45 fps at 1280x1024 in AH with this card, but have to turn some settings down in AH2 to stay at 45.
Before you buy anything, make darn sure your computer has an AGP slot for use with addon video cards otherwise it won't fit in.