Weird Router Connection

My Asus Motherboard recently blew up and I replaced it with a MSI K9A2 CF. Before I changed mobos, my Belkin F5D7230-4 Wireless Router worked like a dream. My wife uses wireless for her laptop, I run on a physical connection passed through the router. No issues at all until recently; it started slowing down significantly when connected. Here's the thing: it runs just fine if connected directly to the modem. If I connect through the router though, I encounter problems with downloading. Downloads start fine, then slow down, then stop altogether and freeze. I can get between 5 and 10 megabytes before it stops altogether. Online diagnostics show that my download and upload speeds, however, are running optimally. I swapped cables, no luck. I bought a Linksys router, my comp won't even recognize it. Did a clean Windows install (wanted to do that anyway after the rebuild), and nada. Installed updated drivers for everything, it still pulls the same crap. Tried disabling firewalls, no avail. My overall setup is as follows:
Win XP Pro, SP2
MSI K9A2 w/integrated Realtek RTL1811 Gigabit Adapter
AMD Athlon 64 X2 5200+
6 Gb PC 6400 800 Mhz RAM
XFX GeForce 7950 GTX

Anybody have any ideas what might be going on here? I'm good with hardware and builds, but routers and networks are a bit beyond my niche of expertise. Any suggestions are very much appreciated, thanks!

 

May want to try an addin nic card. That will tell you if it's your lan port specifically.

 

Have you updated to XP SP3?
Firmware updates on routers?

 

I did update the firmware on the routers, I have not upgraded Service Pack yet, I tend to turn off automatic updates and have had bad experiences with their Service Pack updates in the past (buggy Microsfot releases? NEVER!). I usually just pick and choose which updates I want installed. Given I just reformatted Sunday and I haven't reinstalled much software yet, I guess I don't have anything to lose

I have not tried a new network card yet, the only spare I have on hand is a truly archaic piece from an old, old Compaq desktop, my system wouldn't even recognize it. Worse comes to worse, they are at least and I can give it a shot over the weekend. Thanks for the advice, I'll let you know if SP 3 did anything for me.