As an RF engineer and wireless antenna designer, I'd strongly recommend against modifying your XBOX's antenna circuit unless you know what you're doing.
RF transmission lines and antennas have carefully constructed dimensions in order to allow RF signals to propagate. If you solder some random length of wire on to the traces, you may end up with something that acts as an antenna, but you're more likely to end up with something that performs worse than the carefully designed antenna circuit.
If you're really dedicated to extending the XBox 360's wireless length, here's what you would have to do:
(Warning: If any of this sounds unfamiliar to you, this mod is not for you)
1. Find the copper trace on the 360 motherboard that connects the wireless radio to the antenna. It's either going to be microstrip (thin copper line with nothing around it) or coplanar waveguide (thin copper line with narrow gaps on either side, surrounded by more copper filled with a lot of drilled holes).
2. Use a knife to cut the microstrip or the center conductor of the coplanar waveguide.
3. Carefully strip some small coax (RG174 or 0.141in semi-rigid is what I typically use for these jobs) with an SMA connector on the other end and solder the center conductor to the microstrip or coplanar waveguide center conductor. Ensure that you do not short the center conductor to ground anywhere!
4. Carefully solder the jacket of the coax to a ground point somewhere VERY close to the initial solder point. The goal is to provide as smooth of a transition between the microstrip and the coax, so you want to keep things as small and as close together as possible. Again, make sure nothing gets shorted.
5. Run the other end out of the XBox. I'd mount the SMA connector end in the wall of the XBox somewhere by drilling a 1/4" hole.
6. Connect a 2.4GHz panel antenna (any 802.11b/g antenna will work) to the connector, and aim the antenna where you will be sitting.
Anything less than this is doomed to fail, unless you're 1-in-100 lucky. Also note that this (and any other antenna solution) will only focus the existing energy into a narrower beam. As a result, controller performance will improve significantly in the narrow beam, but decrease significantly behind the antenna, above the antenna, etc.