What Router to use with INTEL PRO/Wireless LAN 2100 3A mini PCI Adaptor?

Since the 2100 is a 802.11b only product (11mb), you will only need a "b" capable router, not a "g" one.  Linksys, D-Link, Belkin, Netgear are the usual suspects.  Maybe search the forums for +Intel +2100 +router and see what others are using.

802.11g is backwards compatible with 802.11b (though backwards compatibility can also be disabled on some access points).

If you can justify the difference and can see yourself upgrading any time in the near future, it may be worth it going with g over b, as both will work, and will give you more bandwidth in the future if you decide to go you need it and want to upgrade.

I can't comment on compatibility yet, as I've not been able to test the Intel with mine (thought I can't imagine there being a problem without me yet hearing about it), the DLink 802.11b series APs are very good (we have two here at the moment, a 900+ and a 614+) - from what I have heard about their ExtremeG products, they're just as good.  You also then have the added advantage that DLink support transfer modes of up to 4X on 802.11b (so 44Mbps) and at least 2X on 802.11g (so 108Mbps)... and they might even support 4X by now (again, not 100% sure).

If you're not using DLink cards though, you be restricted to standard speeds (11Mbps by the 802.11b standard, 54Mbps by the 802.11g standard), but again, it's compatible with both, so you can connect both 11Mbps and 44Mbps devices at the same time without needing to change anything.  You may just lose a tiny bit of signal strength by using 11b 4X though.

Hope this helps :)