Ehrrg... what do you mean by dedicated resources? An onboard NIC is as good as a dedicated one for your average user, and if it needs to be "replaced", just disable the onboard in BIOS and throw in a new one. I've no clue what those "dedicated resources" you're talking about are.
Usually separate NICs are bought by those
-who need more than one RJ45 connector for some reason
-who need to replace a fried onboard NIC
-whose computers are too old to have an onboard NIC.
-who really need high transfer rates (1000Mbps) for a LAN setup, and the onboard is only 10/100
-who buy into fads like Gaming Network Card
If you have a working onboard NIC, don't bother buying a dedicated one. Your internet performance is not going to increase by a slightest bit (unless your NIC has some issues, of course), and for simple P2P LAN (home network) setups 10/100 is more than enough.