Multilink Frame Relay - which IOS supports it

I am using 12.4(3G)

Thanks for the update. My problem is that I'm using a 2621 that doesn't support 12.4. Do you where I can get the (what should be a simple) answer to this question?

Try this:

aof#sh ver Cisco Internetwork Operating System Software IOS (tm) C2600 Software (C2600-J1S3-M), Version 12.3(17a), RELEASE SOFTWARE (fc2) Technical Support:

That router is currently configured for Multilink PPP....

Feature Navigator knows. :)

(But then again, you are using a router that has been announced end-of-life over 5 years now, and the official end-of-support is in a few months).

Looks like you need IP+ and up in 12.3(x) or 12.2(x)T. Looks like there is a few images in there that can deal with 16M Flash, but you'll need the max 64M of DRAM to make it work.