5x32mb cache sata2 500gb raid5 drive array upgrade suggestions

Hi. First time posting, but frequent reader. Love the website.

I currently have an Asus m3n78-vm mobo in my media pc. I'm running 5x500gb 32mb cache seagate drives in a raid5 setup. I'm currently getting 400mb/s transfer rates  

I know its sick, but I only use my array for a windows OS, xp media center. I store all my data on a 2tb nas box.

I want to go faster, just like in my car.

I want to keep using my onboard raid controller. I'm also considering using my E-sata port for a 6th drive... but I'm somewhat worried about the 2TB ceiling for XP...

What can I do to upgrade? I'm currently looking into swapping all 5 drives for 5 raptor 10k 74gb drives... they're decently priced in the used market right now, imho. Or 6 raptors.. I won't hit 2TB this way!

SSD would be nice, but somewhat pricey right now.

I don't have an unlimited budget, but I can sell my drives and chip in some cash.

Does anyone have suggestions?