Internet/Network traffic slow through NIC - Normal wirless

I just built my first computer and have had it up an running the last few weeks. One thing that I'm not able to completely figure out is my network connection.

My comp has a Gigabyte motherboard with 2 ethernet jacks. Regardless of which jack I connect to (from my Linksys 160n router) my hard wired connection is much slower than if I connect wireless.

For example if I'm downloading a file (from the internet or on my home network) it will struggle around 1Mbps when hard wired, but will zip along when connected with a wireless adapter. The weird thing is that my wired connection is pretty fast at first, but seems to slow down the more I use it.

For reference I'm currently using Windows7 (64x) RC as my OS (I have a coy of XP, but was hoping I could get it up an running with Win7).

Any ideas? Not sure what I should be doing.

Thanks!