As Jorgemario descibed in his post, some host NICs can work with some switches in a port channel configuration. Not all NICs support this. Even when they do, when you port channel, you should also chose your hashing option to best distribute your traffic. Also note, single flows generally will only use one link.
Some host NICs support multiple NICs with special modes. One I recall was only one port would receive data but all others could transmit data. (This worked well for servers that sent more than they received, e.g. typical web server.)
Assuming your server NICs are already running at gig, also keep in mind, gig is getting close to typical performance levels of some disk I/O subsystems or the system bus. I.e., if you want to jump into multiple gig (or 10 gig), other server system components need to ber able to perform at that rate too, not just the NIC.
Another option to consider is moving to 10 gig, but then you are into new hardware.
All the above assumes you only need to increase bandwidth to one very busy server. However, if you have lots of really busy servers, all running with gig, it's possible you'll bump into the bandwith limitation of slots and port groups. Jorgemario also touched on this for port channeling, but even with single port server attachments, you might carefully consider how you use the ports to insure a server always gets its port gig's bandwidth.
For instance, using a 4548, you might only use one port of a group of eight to guarantee that port full gig (not shared). Or perhaps have your very busy server on one port of eight but other ports, in the same group, attached to very little bandwidth demanding hosts.
