Couple of things:
1) You should be very sceptical about wifi speed claims. They ignore overheads and are essentially theoretical maximums under lab conditions. If you get a third of the published "up to" speed in the real-world over wifi you're doing very well.
2) Are you sure it's Cat5 cable and not Cat5e? It's been a long, long time since you can buy the non 'e' variant of Cat5. Cat5e is rated for a gigabit at up to 100m.
3) Even if it is ancient Cat5, it's still likely capable of running a gigabit. Have you got it plugged in to an ancient switch or router that's only 100mbps? Even a 30-50m run of old Cat5 would likely handle a gigibit.
Failing all of the above, you could configure the multiple network adaptors, but it requires messing with routing tables. It's not rocket science but you can make a mess of things so I wouldn't recommend it unless you know what you're doing.
What's the actual problem you're trying to solve here? Have you got lag in games? File copies taking too long?