How effective compression can be is always dependant on the type of traffic you have, plain text is highly compressible whilst image files or .zip files will not compress at all (or may end up bigger). Therefore it is hard to predict how much extra bandwidth you will gain.
However enabling compression can have a negative impact on the CPU on your router and can end up degrading performance not increasing it, the higher the link speed the higher the potential hit on the CPU (the 2600 is only rated at 256K for compression).
I would recommend using the hardware compression module (AIM):
http://www.cisco.com/warp/public/cc/pd/rt/2600/prodlit/dcaim_ds.pdf
The AIM-COMPR2 ($995 list) is rated at 8Mb with a 4:1 compression ratio but this will be End Of Sale in September.
The AIM-COMPR2-V2 is only supported on a 2600XM but is rated at up to 16Mb but still with a 4:1 ratio.
http://www.cisco.com/en/US/products/hw/routers/ps274/products_data_sheet09186a0080091b8a.html
Alternatively look at external compression boxes such as Expand which can provide very high compression ratios for certain applications such as Cirtix
http://www.expand.com/
