Well when it comes to graphic you can purchase any card on the market (there are links to some below) . All PCIE graphic cards are backward compatible for all PCIE version (wich are 1, 2, and 3). So long as you have a power source that can run it.
The only problem I see with the optiplex is that the graphic card might not fit. Any good graphic card this one for example:
e.g.
http://www.amazon.com/...00DBPKEOI/ (for a list look bellow)
Takes up the room of two PCIE slots. Your computer only has one. I work for dell fixing there laptops and desktop, I've seen this on a few times and I don't believe a gaming graphic card will fit in that case. If you are super dedicated to this though there are a few options:
Buy a new case
Make a new case
Take it out of the case (this is an option I ran a computer on a stool for 3 mounths)
The Vostro on the other hand has plenty of room.
In the case that you are didicted here are some links that will give you graphic card options at differant price ranges
300$ - http://www.tomshardware.com/answers/id-1836271/300-dollar-graphics-card.html
400$ - http://www.tomshardware.com/answers/id-1753260/graphics-card-400-dollars-7970-gtx-770-gtx-
680.html
500$ - http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-5.html
600$ < - http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-5.html
A good 800 watt power supply will power all these for about 100$ bucks.
e.g.
http://www.amazon.com/...zon.com/s/
When it comes to the mounting brackets for the Vostro you will never find them sold on their own.
Either buy a broken one for salvage
Rig one up
Just leave it loose it won't hurt it
