the maximum length for a cross over cable ( or any ethernet (cat5)) cable is roughly 100meters,
If you want to share the internet then a router probably is best, unless you already have a router based modem these can be expensive to buy.
There is a chance that you might have a regular modem with a network adapter, and you need a computer to sign in... your modem may have the choice of Ethernet or USB.
in this case, I suggest (as a cheap option), rather than buying an 80 quid router go and grab a freetalk box from dixons, this only costs about 20 quid and can be used to sign onto the internet, combine this with the cheapest ethernet hub you can find and you've got an 80 quid router for about 30 quid, (that's what I do),
my setup is Telewest modem, connect via Ethernet to a freetalk box, which is connected to a hub,
The free talk box has built in firewall, DMZ, Routing, DNS and DHCP server.
oh and it's also a phone, with a low cost subscription and a free to any UK landline phone!