The advantage of getting another gigabit switch is that if all the LAN party computers are connected to this switch and want to transfer large files between each other, then that will be fast, and not affect computers connected to the other switch.
As you mention, if these LAN party computers are transferring large amounts of data to/from your server then that gigabit connection could become saturated, assuming your file server is able to handle that I/O transfer rate.
I'm not quite sure what you are trying to achieve, but a good place to start looking for switches would be on a well known auction site.