Unless there is some reason you must have an internal modem, I'd go with an external. It's going to give you better performance and stability. Most external serial modems will work with Linux. We know the Actiontec serial and the Best Data serial work right out of the box in most cases. I don't think x86_64 would be a factor with either.

Take Dan's advice. I struggled some time ago with this (although things have improved with Linux support somewhat) until I got broadband. The Motorola SupraExpress external serial 56k works (the PC model, not the MAC model), as well, but they are hard to find. I would go with Dan's suggestion.
 
If DSL becomes available in your area, consider this as an alternative. I know, but they have been working on DSL to make it a more viable option (switch station and booster ability), so you may see it come to your area in the near future.

Best Data Best Data 56k v.92 model 56SX-92 rev2
 
 
I sure wish the hell Id seen that I could have got it for less that 16 bucks
as aposed to the 35:)
 
 
Well yes it will dail out on my linux's BUT it didnt hold connection long
In fact on my fedora it would only seem to post a 9600 bps
so after a hour I gave up.. And put in on my Mandrake.. It only faird better , But still woiuld die after few min,
It works well on windows LOL..
 
 
Im useing a Old 3com rockwell modem it seems to work well with both destros.http://www.linuxcompatible.org/images/icons/icon_wink.png
 
 
So its eather the ISP that dont mesh well with the best date modem on a linux ..Or something else.. ,That im clueless to..