WIC (WAN Interface Card) slots are small slots that accept interfaces that are primarily used for WAN connections, e.g. T1. As I recall, there were some ethernet WICs that could be used in the 37xx routers, but the actual throughput is limited in the WIC slots compared to the NM slots.
NM (Network Module) slots are larger than WIC slots, and have a higher throughput. The NM cards generally have more processing power than WICs, and can offer more LAN-type interfaces.
The 37xx series of routers went EoL (End of Life) six years ago, so Cisco no longer supports them. You will need to use a newer, larger router if the 3725 doesn't meet your needs. That is why Cisco has different models; you can choose the model that best fits your needs.
You could always use logical subinterfaces on a physical routed interface to get more interfaces for experimenting in GNS3.