To aggregate multiple network cards together into a single pipe to increase bandwidth to a network, you need a switch that supports Cisco's FastEtherchannel standard. Instead of assigning ip addresses to the NICs, you use trunking software (may or may not be included with Win2K server) to create NIC pool. Next you connect to your switch (for example, my Cisco 2924XL has a web interface) where you then assign specific ports to a FEC pool.
Without a switch that supports FEC, you are limited to basically three options:
1) Have traffic return over the interface it was received. By default, all traffic gets returned over the default NIC. Which means even if you map a server volume via the 2nd NIC ip address, all traffic still returns over the first. There's probably some registry setting or routing option to force return traffic via the source route -- I haven't looked into how to do this yet under a Windows environment but I currently use this technique on Solaris servers so I know it's possible. Note, this is only useful on the server-side -- when you initiate a connection, you always go over the default route. In a server situation, you can use DNS (or other techniques) to rotate usage between NIC #1 and NIC #2.
2) Have direct connection to another computer with multiple nics using crossover cables. Without a hub or switch to direct traffic, trunking software on both ends can properly recognize multiple nics as a single connection.
3) Use DHCP/write a script/etc to periodically switch the default route. How much use is this? Not a lot -- some network applications are rather sensitive to route switching (for example, irc) and just would piss you off after a while.