I am using an 8600GT with 512mb ram and an 8500 with 256mb ram, and can see no difference in the performance. I am ordering a MB right away for a new MD that has a 8200 on it, and would be interested to hear of others opinions on the merits of these model differences. On that note, many use 6000 & 7000 series cards in HD 1080p modes with success, so I doubt the difference would be noticeable.
On the issue of network performance. I would have to strongly recommend a wired connection, due to simplicity of PXE boot from bios of MD clients, bandwidth, and wireless flakiness sometimes. I recommend gigabit (although not necessary, I have 10/100 and it works fine) due to it's now cheaper prices and availability of most MB/systems with gigabit onboard. Wireless for an orbiter is fine. And I've heard that up to 7 streams of media can be accommodated on a dedicated 100mb pipe. So 54mb's (or more like 45mb's realistically) of 802.11g would likely be fine, if it were pretty reliable. But the complexity of configuring PXE boot of a wifi card could be prohibitive.http://wiki.linuxmce.org/index.php/GRUB_PXE_network_boot