If the built in nics are on the pci-e bus you have less to worry about, but even then 2 gigabit ethernet ports will usually work quite well over 127MB PCI bus, as a single gige port will only use at most 90-100 of that (if you're lucky), and unless you do massive file transfers on both all the time it won't saturate and if you only used them for internet there's nothing at all that will max it. Unless you have a couple DS3's at home
I use the second port for my gige connection between my 2 main machines and my HTPC so they have dedicated connections for video transfers. They just use a different subnet over the regular lan so as to not interfere. You do have to watch out and not set a default gateway on both, windows hates that. You'd have to use a third party app to get bandwidth sharing for two internet connections. I use a pfsense firewall for my two cable modems and I could setup load balancing, but one is for my internal lan, the other is for my website and email servers.
Most load balancing doesn't give you the bandwidth of both at once, it divies out connections, one connection to the first modem, the next to the second modem, etc. So if I load balance my 8mb and 3mb modems I won't get 11mb available for a download for example.