I was just telling that if a XM platform is available the IOS 12.4(24) is a further option. In my experience I have found the 12.4 train much better than 12.3. And then... Yeah, I was telling about memory capabilities because the 2600xm platform differs from the normal 2600 only for the amount of memory you can install.
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For MPLS Support on 1800 & 2600 Series you need IOS Release 12.3(14)*T or later.
Cisco IOS Release 12.3(14)YT on 1800 Series ISR
Cisco IOS Software Release 12.3T Features
According to the Cisco Feature Navigator, it requires a 2691 to run MPLS.