This should probably go in the "This and That" section but anyways, we would probably need a little bit more on the system specs. The MX400 series of cards are not that great (my old Kryo II would out perform this card). Nvidia cut most everything off of those cards. I looked at the MS's site on Halo and the card is supported, but it looks like it's at the low end of the spectrum. Might want to upgrade that thing to at least a 5200 or 6200 or get a ATI Radeon 9250 for around 50$ if your strapped for cash. Could be why your refresh rate is stuck at where it is, I don't know. Have you updated drivers on it lately might want to try that if you haven't.  There is also game update V1.07 out on Halo's site. Might want to install that as well if you haven't. Hope this helps out some.   

Mimium game specs:
# Microsoft® Windows® 98SE/Me/XP/2000
# PC with 733 MHz equivalent or higher processor
# 128 MB of system RAM
# 1.2 GB available hard disk space
# 8x speed or faster CD-ROM drive
# 32 MB T&L capable video card required

Support Nvidia cards:
•      GeForce 256 DDR
•      GeForce 256 Ultra
•      GeForce 256 Quadro
•      GeForce2 MX
•      GeForce2 MX DDR
•      GeForce2 Go
•      GeForce2 Quadro
•      GeForce2
•      GeForce2 Ti
•      GeForce2 Ultra
•      GeForce2 Quadro
•      GeForce3
And up

Yeah, that's probably the case. Halo only sees that it's a 866 and not a gig so it's locking you out, at least it's what I'm guessing (don't have the game although I've been meaning to get it).