Slow download speed on single network computer

I recently built a fairly powerful HTPC capable of playing 1080p/DTS movies without a hitch. However, I've been experiencing very slow download speeds and unbearably jittery streaming video from sites like youtube and hulu. I decided to check my download speeds using Speedtest.net and found the problem. My desktop is hard wired into the router in my office, but my other computers were all wirelessly connected from my living room for the speed test. Here are the speeds I achieved:

Desktop - 7.41 download, 0.72 upload
Laptop - 7.39 down, 0.73 up
iPhone - 4.77 down, 0.70 up
HTPC - 0.65 down, 0.73 up

I can't figure out why I'm getting such slow download speeds on my HTPC. The network signal on my HTPC fluctuates between fair, good, and excellent. My laptop says it has "low" signal, yet it has no problems downloading. The HTPC uses a Linksys USB dongle to access the wireless G network. I'm using a Linksys WRT54G router with the latest Tomato firmware and have boosted the signal slightly to improve reception.

Does anyone have any advice? Thanks in advance!