Router or Brouter

Cisco 2600 will do fine. It's also very easie to expand this one for the future when you need to.

http://www.cisco.com/en/US/products/hw/routers/ps259/index.html

(remove any spaces in the url)

The solution depends on your budget and volume of traffic between two networks. Cheapest solution is:

1. Install additional NIC card to one of your servers (if you have new servers, they usually have 2 NICs preinstalled).

2. Configure second NIC with IP address on second network.

3. Enable "IP Forwarding" (in case of Windows NT), or configure "Routing and Remote Access" (in case of Windows 2000/XP).

2. Second option is in using Layer3 Switch. Generally speaking it's switch with limited Routing functions. It does not allow you to do a lot, but it will switch packets between networks.
Now many manufactirers allow you to use switches as simply routers (At least, Cisco, 3Com, Nortel, D-Link, etc.) You may even connect your serversto the same switch (for example, configure one half of switch as one network, and second one - as another). This will also improve your general network performance.

3. Use real Router. It could be Cisco (yes, it could be 2621, 2651, etc, but it will cost you 3000 USD at least). Again, depending on traffic you may use lower class Cisco Router (i.e. 1605 with dual Ethernet - ~1000 USD). If company allows you, you may try to find something cheaper at eBay. Another way to look another manufacturer's products - search "dual ethernet router" at Google. I have found at least 15-20 products which allow you to do what you want.

If you have more questions, send me e-mail.

Good luck.