You can buy two wireless transmitters and a wireless router and then you can network between the two computers and also share the adsl connection. But I'm not so good in network problems, so I hope someone can back me up.
700ft is too much for a single CAT5 cable as well. At least for 100Mbit (not sure about 10Mbit). If you use directed antennas for the wireless you could get the range up quite a bit, right? Even 700ft perhaps?
The specs for a single run (without a repeater) of CAT5 UTP is 100 metres, and there is a margin for error.. that is at 10 or 100mbit. I think you can go a little further using coax.
However a repeater is very cheap.
Since you are in different buildings, I would go for a more high powered wireless device.. theres a few different types, but I don't know much about them.
haha! if you 2 live on the same block but diff. building, just get a 700ft cable and extend it to his place. extend the cables over on the roof. i know someone did that b4, plus thats the only option you got.