Hi freefree,
I'm more or less in the same boat as you except I still didn't buy my vidcard yet.
My motherboard is a Crossfire ready model so naturaly going ATI would take better advantage
of my board but I really haven't found anything about Maya's performance with x1800 cards (or
x1900 cards for that matter).
That being said, I'll post my specs and share what I've found out so far.
- ASUS P5WD2-E Premium board
- Intel Pentium D 940 dual core CPU
- 2GB (2x 1GB) DDR2-800
Most of my findings reveal nVidia cards are heavily favored for Maya.
The main reason is because of poor ATI drivers support. But in my humble opinion this is rather
relative. Most of the problems people reported using Maya with ATI cards were with the older
cards like the 9700, 9800 models. These are a few years old and I'm certain ATI came a long
way with their drivers support noawdays.
Unfortunetly I haven't found anything on people running Maya with an x1800 card 
As for your Crossfire question, the only way I know you can boost the performance without adding
a second card is if you overclock. I wouldn't really advise that unless you've done it before and really
want squeeze everything out of your games. The x1800xt is fast card as is.
Hope it helps and hopefully, a Maya user using an ATI x1800 card will see this thread and give
us some input 
Have a good one!