depending on your hardware.. If the onboard LAN chip happens to be Realtek, save yourself some trouble in future and disable it in BIOS in advance. All sorts of mystic crap can happen with it. Also, Realteks really like to hog/strangle your systems CPU. Also disable sound and anything else you do not need.
Use preferably cards from Intel, even if these are desktop cards and not server cards. Less issues.
When your are using multiple cards on a PCI slots, make sure you are not exceeding PCI bus throughput (couple of PCI gigabit cards could achieve this). The throughput varies depending on bus speed and if the slots are 32bit or 64bit. Lowest value is 133Megabytes/s, highest 533. Generally 266Megabytes per sec.
Older CPU-s might be too weak for handling high speed links, 1GHz+ recommended
You got AMD X2 3800+. So it's either S939,AM2 or AM2+.. too bad. If you had s754 motherboard you could have bought S754 mobile Turion64 ML/MT from Ebay and use it, Turions often worked on desktop Nforce4 boards,they support 64bit and have power a plenty (comparable to P3M) while not eating too much power..(25W/35W max TDP). One of the few possible options while trying for low-power system on old-tech.