1721 router query

Hi,

I hope this is a good place to ask this question:

I have two Cisco 1721 routers.   I am running IOS 12.4 / 12.3 (memory
constraint on one).  Both are Ethernet / Ethernet using WIC-1ENET cards.  I
am looking for a way to improve performance; can I change the WIC-1ENET
(10baset) for a WIC-4ESW and run instead at Full-duplex / 100Mbps?  Are
there any considerations e.g. like IOS version?  If this is possible, do you
think this is a good idea (I have read that there may be a 10Mbps limitation
on the processor?)?