|"Ignoring the price, what would be the absolute best I could get my hands on that would be compatible with my Pavillion 503n's power supply?"|
You don't understand. Your power supply isn't the problem, the fact that you're stuck running a standard PCI card is the problem. The older AGP bus & new PCI-express bus are dedicated to graphics & run at 66MHz & 100MHz respectively. The PCI bus only runs at 33MHz plus it is a shared bus. No matter which card you install or how much you pay for it, you will NEVER get the bandwidth needed to run games decently, even on the absolute lowest settings, unless you're willing to deal with low frame rates, lag, stuttering, etc? IMO, you're throwing your money away. It doesn't matter how deep your pockets are. And if you have that kind of money to toss around, why not invest in a new board & video card or better yet, buy (or build) a modern system that will hold you over for several years to come? Your system is about 8 yrs old, it's about time to let go.
Here's your motherboard specs: