Yep, that is a concern.
I know some of the Dell boxes have non-standard PS mounting holes and cutouts. I wonder if the XPS 8500 can take a non-proprietary power supply?
AFAIK, all of the Dell Minitowers use standard ATX Power Supplies. I upgraded one in a little Inspiron 530 with a Core 2 Quad (Q6600) it a while back, and just checked customer reviews of PSUs at newegg.com to make sure it would fit into the case OK (as some are a "tight" fit), finding one that others said worked in it.
For example, searching for the XPS 8300 in the Customer Review Section for Power Supplies at newegg.com yielded 3 results of users that bought PSUs for the XPS 8300 (which is the same case size as the XPS 8500).
http://www.newegg.com/FeedBack/CustratingAllReview.aspx?DEPA=0&N=40000058&Order=0&Pagesize=&Page=1&Keywords=XPS+8300
But, it's my understanding that the 460 Watt PSU in the Dell XPS 8500 has what you'd need for most video cards anyway, with dual 6 pin connectors, and Dell saying they will handle video cards drawing up to 225 Watts (which means pretty high end cards would work in one).
Dell sold the XPS 8300 with cards like the Nvidia GTX 560 (and I've seen some sold with the 560Ti when looking at how refurbs were equipped, but I think they used an OEM version), Radeon HD 5870 and 6870, and I've seen previews of the XPS 8500 at a review site where they shipped it with the newer Radeon HD 7870.
I'd just make sure the video card you buy will physically fit into the case (as some are longer than others), and I'd probably stick to one with a max power draw of around 175 Watts to stay on the safe side (even though Dell claims the PSU in the XPS 8500 will handle video cards pulling up to 225 Watts).. If you look at card specs on AMD and Nvidia web sites, you'll usually see max power draw listed in a card's specs.
Looking through some of the card reviews, I've seen users mentioning getting cards to fit into the XPS 8300 up to around 11 inches in length, although they had to move the PSU and hard drives out of the way temporarily first, they still got them to fit in.
So, if you wanted a real "screamer" of a card, I'd look at the new Nvidia GTX 670, as it draws less power compared to the older 570, and most should be under 10 inches in length, too. See specs for it here (max power draw of 175 Watts):
http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-670/specifications
It was just recently announced, and appears to be almost as fast as the new GTX 680. I just don't see where you'd need anything faster, even for gaming. It's a hot seller, and although newegg.com had some in stock when they were announced, they're all sold out now. LOL
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600315498%20600311820&IsNodeId=1&name=GeForce%20GTX%20670
But, I don't see where you'd need a card anywhere near that fast for most anything you'd need to do. Personally, I just could not see myself spending approx. $400 on a video card for any reason. But, to each, their own. ;-)
IOW, if I really needed something faster than the cards Dell offers and wanted Nvidia instead of AMD, I'd probably stick with something like an Nvidia GTX 560SE (a little better performance compared to the stock GTX 560 cards you find, yet power draw requirements are still lower than the GTX 560Ti), for around $129 (pretty good "bang for the buck" from my perspective). I'd probably go with this Zotac card (so that I'd have a DisplayPort, too.):
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=nvidia+gtx+560SE&x=0&y=0
--
JimC