Actually, that's what I remember as being the big problem with the FX5200 - it can do DX9 stuff, like advanced pixel shaders. So when you fire up Halo, it bumps things to the highest supported pixel shaders - but the card doesn't have the horsepower to handle the extra graphics. So you get massive amounts of lag, and virtually no increase in any kind of actual processing power from the MX440.
I could be wrong about that though. However, I also remember reading in reviews that the 5200 isn't really worth it.
If the 5200 doesn't have OpenGL mode, that's just another reason to go for the MX440...I'm pretty sure it could do hardware accelerated OpenGL, and OpenGL is actually more stable under fs2_open right now.
However, $30 for a graphics card is pretty limiting so I doubt you'll get much better short of going on eBay as CP suggested. ('s not a bad idea, you might be able to snag a card from someone else upgrading to a GT or to PCI-X.)