This might seem to be a radical suggestion, but if you can avoid putting costly interfaces in your routers and possibly having to upgrade other components as well, and IP is the only protocol you're routing, you might want to try L3 IP switching instead.
Let's look at it first from a cost perspective.
Never mind that you might have to upgrade the NPE on your 7206, let's ignore that for a moment. Strictly speaking, you would need a PA-GE and a GBIC-LX/LH (a.k.a. WS-G5486=). That's $6,000. plus $995. list price; total for the 7206, if the rest of the router is in perfect condition to accept the port adapter: $6,995.
Same caveats, dealing with the 3640. Let's assume you just need the interface. NM-1GE plus GBIC as above. That's $4,500. plus $995. list price; total for the 3640: $5,495.
Total for the two routers: $12,490.
Contrast with two Catalyst 3550-24-SMI switches, which will route IP (with RIP and static routing protocols only, for SMI software). $2,995. list for the switch, plus let's say an LH/LH GBIC at $995. and an SX GBIC at $495.; total for one site: $4,485. Times two sites, equals: $8,970. Less expensive.
And if you want the EMI version of the software on the switches, for high-end routing protocols like OSPF, EIGRP, and IP multicast routing, add $1,995. per switch, or $3,990. for two.
Total for the two switches, with EMI software load too: $12,960. Just a bit more expensive than doing it with routers. But you get more with the switches.
For that extra $470. list price, the switches get you 24 ports of 10/100 Ethernet at each location; the routers don't do that for you.
Now consider performance.
It is likely that you would have to upgrade elements of your 7206 to get the kind of IP throughput that you'll get on the switches, or to even be able to use those Gig interfaces at all; which only helps the switches come out cheaper on overall cost. Even more so if you only use the SMI software and can get by with static and RIP routing.
Granted, there are things the routers will do for you, that the switches cannot. Access lists, maybe? Handle WAN interfaces? Hey, I like routers, don't get me wrong.
But if all you need is lightning-fast speed between the two sites over the fiber, and IP routing for broadcast domain containment, then (L3) switch wherever you can, and route (with a router) only when you must. Generally speaking, you'll get better IP routing performance out of the switches than you will from the routers.
Part of my job is getting customers to see how they can use dark fiber. A lot of people come from the leased-line mentality, and view the use of "wide area" fiber the same way. Most are leasing the fiber. But whenever you have the ability to use dark fiber, you should approach it from a "campus switching" perspective.
I really hope my rant here made some sense, and that it helps.
