7500s? In 2004? Throw those things in the trash where they belong. It's always amazing to me how many people will cling to obsolete things for years just because it is what they know.
Even a Juniper M5 will do 16 OC3's with line rate filtering and forwarding. There are probably a dozen design considerations based on requirements you haven't described, but if you're doing primarily sonet, 7600 isn't really the way to go.
I usually agree with RAS, but not this time. 7500's have a place; not everyone is looking for wire speed at 2499384 gigglebits. In our network, 7500's have made a home for the leaf-ends of DS3's in dial and DSL pops, handling 20 to 40 megs/sec with easy (ie, vip2-50's at 15% CPU). Also handle MPLS AToM with ease, PPPOE/A if needed (we choose to segregate that onto VXR's). 7500's also can handle many ChDS3's with ease. And, also, with RSP16/VIP8, lots of traffic can be handled. All at a substantially less price than even the cheapest used M5 you can find. 7507 + dual ps + rsp4 can be had for $1000 to $1500, and VIP2-50's can be had for $300. And then, you can use all the PA's you have laying around from your 7200's. Secondly, 6509 + OSM is actually a sweet solution, and provides way more LAN aggregation than any traditional 'router' can. Wire speed, too. (disclaimer: this is coming from someone who has all m5, m10, m20, and m40 core, with 6509's, 7500's and 7200's strewn all over his network. These are my opinions, and probably differ from people who are indentical to me).