Sony Vegas Movie Studio 9.0 HD Video Preview Lag?

When ever i preview my HD videos in sony vegas movie studio 9.0 hd the preview quality is VERY laggy. I have tried lowering the preview quality and that did not help. I also just purchased a EVGA GTS 450 OC graphics card, which i thought would fix the problem but i guess not. I know the SHIFT + B trick and I was wondering if anyone knew any different methods because SHIFT + B takes A LOT of time.

Also, my computer is a custom built AMD Phenom 2.2ghz quad core, with 4gb of DDR2 RAM, 250GB hard drive (5200 rpm), and the newly added EVGA GTS 450 graphics card with 1gb of GDDR5. I would think this is enough power to edit HD video but i guess not?

So, does anyone know how to get rid of this lag? Is my computer not fast enough?

Thanks!

Really one of the best device of sony to watch movies. It's a great name in the industry and it keeps promis everytime.