HP 6531p Desktop: Upgrading From Onboard Video Card

I'm looking to upgrade the video card on my desktop, and while I've done this in the past with prior computers I always just kinda "winged it" and lucked out. I did a bit of researching and was wondering if any of you hardware buffs might be able to confirm my findings, or steer me in the wrong direction if I'm wrong.

I should say that I'm not a PC gamer, nor do I want to be; my goal is simply to upgrade from my onboard ATI Radeon HD 4200 to something that has dual DVI outputs. My onboard video card supports my dual monitor setup, but only by having one plugged in via DVI and the other plugged in via VGA.

My desktop is an HP p6531p (link), with what looks like a 250W power supply. I'm finding most video cards, such as the EVGA GeForce GTX 550 Ti (link) require 400-500W power supplies, which means I'll have to upgrade that as well.

Knowing nothing about power supplies, I did find a forum post online (link) of someone with my same desktop who did upgrade his power supply to a OCZ ModXStream-Pro 500W power supply (link), so I'll probably just go with this since I know it works.

So here are my questions:

1) Is there any reason the OCZ ModXStream-Pro 500W power supply might not be the right choice? They offer 600W and 700W versions as well; is there any real reason for me to spend the extra for one of these?

2) The EVGA GeForce GTX 550 Ti looks like it's what I think they consider a "dual slot" video card, meaning it will take up the room of two cards in the case, even though there is just the one slot plugging into the motherboard. Right?

3) Does anyone have any recommendations for a non-dual slot video card that will support dual DVI outputs, just in case for some reason I discover my case doesn't have the room for a dual slot video card?

4) Anything else I should know or plan for?