I cannot find NIC cards that have more than 1Gb uplink,
10 Gb/s Ethernet cards are a few hundred dollars, and 10 Gb/s switches are less than $200 per port. Inifiniband and similar go to 40 Gb/s or even 100 Gb/s, at substantially higher costs.
but then load balancer is a busy box, isn't it?
That is correct. I'd expect to pay about $50k per box from someone like F5 or Juniper, and you typically want two, so you can run them in "hot fail-over" mode, where if one dies, the other one already has all the state to keep running the service.
I also recommend actually testing this fail-over under load, as some vendors may be promising things that don't actually work for real.
do I not understand what load balancer perform ?
The good news is that the load balancer typically doesn't need to process the actual data or application state, it just needs to re-write and forward packets using reverse NAT. This is a lot easier than running some Java application or interpreting some PHP script or whatever.
Try this for starters: F5 article on load balancing.
Can I ask for 1 Tb of bandwidth
If your machines are in a data center where some first-tier back-bone providers provide peering, that may be possible (you'll probably get something like six 192 Gb/s links, rather than one 1 Tb/s link, though.)
High-quality bandwidth could be budgeted at approximately $10 per Mbit/s per month, so if you want a 1 Tb/s 95% reservation, you should expect to pay about $10,000,000 per month. Given the high volume involved, you'd probably be in a pretty good position to negotiate for substantial discounts, though.
However, given the nature of your questions, I'm not sure that you're quite ready to start a business at this scale. Could you describe a little more about what you're actually trying to do?