You aren't pushing them limits of the wired network by any stretch. I would guess your bottle neck is your PC. Nothing you listed network wise should be an issue. I am streaming MKVs to my Xbox (which we all know is 100 base) and I've never had an issue. Do you have extra switches in the equation?
I realize my network is plenty sufficient and other than my PC to TV Tuners connection the rest is all 10/100. As I sometimes record several HD programs at once 100 would be insufficient for that leg but the rest is fine on 10/100 since the Xboxes can't connect any faster anyway.
As to the PC, it's a couple year old quad-core processor with 16gb of RAM running Win7 64bit. It doesn't seem to matter if I'm recording no programs or downloading any files or if I'm recording 4 HD programs while downloading larger HD video files ... perceived performance remains about the same. While streaming none of the cores exceeds 20% (highest at 18/19% and the other three will hover around 10%) and the RAM is always below 50% so I'm not thinking the PC is the problem.
Switches: There is a 4 channel gigabit switch I added next to the router (ground level, in the office) to take the tuners and PC off of the router LAN ports to see if that was the issue but it doesn't seem to affect anything so this is why I was thinking it may not be the router but I'm not sure what else it would be. I do have one other switch in the basement that feeds the rest of my home network and it's a D-Link 24 port 10/100 switch. It's been a rock solid unit but maybe it could use a power cycle or something since it's been on for about 7yrs straight. I don't feel like this is my issue either though but at this point I don't know what to suspect other than the router handling traffic.