I've noticed that when I go near Oblivion Gates my framerate drops and the game starts going slower.

I'm wondering if anyone can lead me in the right direction to optimize my PC to work better.

Here are my specs:

Win XP SP2
Intel Core 2 Duo 3.0 GHZ E8400
4 GB RAM (says somewhere around 3 is usable)
ASUS EN9600GSO TOP 384 MB Videocard
Echo Audiofire4 soundcard
Antec Basiq 550W
Samsung 206BW 20" LCD monitor 1680x1050 resolution screen.

If you upgrade for Skyrim (HD 6670 for $59 after rebate), that will reduce the effect, but it will still happen some, even with a High End GPU.