The only benefit, will be the knowledge gained in working on setting it up.
You are capped at the modem to whatever you are paying for (or by the line...), so the bottleneck 99% of the time, is the isp, not your *connection to the modem/switch*. So if you have 4 1000 mbit ports in the modem, and you have a 40 mbit limit on a 1000mbit WAN port(up and down...for giggles not because you actually have that)...40<1000...40<2000...What ends up happening, is if you split the two lines up, and "load balance" as you call it, it ends up freaking the switch out a bit unless you have the ability to tell it that you are "load balancing", which most(all?) modem/switches do not. It creates overhead both on the computer side, and on the modem/switch side. Realistically you aren't going to notice that overhead because you aren't able to notice it (0.5ms vs 0.7ms for example), but you can quantify it if you had the right tools.
If for some reason you actually have a 1000mbit connection in...Then the above is still true. If your connection is somehow greater then 1000mbit, then yes, having two 1000mbit connection in to your pc would help you (assuming you can get the teaming/load balancing working).
On the other hand, if you are doing this for people on your LAN, then there is the possibility of using more then 1000 mbit at one time. There are a some nuances to this...such as the peak usage of a single pc (assuming it also had 2 nics in teaming er load balancing)...would it be maxed to 1000mbit or 2000mbit downloading from the server, the answer isn't always simple. Depending on the hardware, it may peg out at 1000mbit max per connection, but with two downloads, could hit a peak of 2000mbit. Anyway, this wasn't your question, but...oh well. ;-)
Ultimately this comes out to simple math. If your internet connection > then your Ethernet connection, then yes it makes sense to set up load balancing with a second or more nics, otherwise the bottleneck is the internet connection.
You can* peg out the Ethernet card before pegging out the total throughput of gigabit...but thats not worth getting in to. Unless someone is targeting you for some reason, or you have faulty hardware you don't have anything to worry about...
edit: Oops, sorry when i looked at this i thought there was only 1 response. ><...Also edited a couple things above...