I turned my Dell T3500 into a gaming PC, works great, yes you can.

An i5-2400 should be fine for gaming, however, it does depend on the games you plan to run. Could you list a few games in mind? Heres some GTA V,Fallout 4,Call of Duty IW,Witcher 3,Battlefield 1 

I think your best option for a GPU would be a GTX 1050Ti, its $20-50 more than the 750Ti and GTX 660. An R9 270 would require a better PSU I think, whereas a 1050 could run off the slot(possibly, that PSU is very weak). I think a EVGA 500w PSU would be worth it(mine has two 6-8 pin connectors).

A SSD will be expensive, what I would do is use 2 drive setup. SSD for operating system,applications,etc. HDD for storing your games,large files. I run a 240GB Sandisk SSD/750GB HDD in my laptop for those things. If you don't plan to do much with your main HDD, you could get a cheap 120gb Intel SSD/Samsung/Sandisk.

New RAM is a bit expensive, I would check eBay for a used pair. I've bought used RAM and have had no problems. Very cheap too.(make sure the seller accepts returns if you can't use the RAM)

Parts:

• GTX 1050Ti: 2GB Version and 4GB Version (May require better PSU)
• EVGA 500w PSU: Here
• Sandisk 120GB SSD: Here
• DDR3 RAM: 2x4GB and 2x8gb($50-60 more)

Hope this helps!

Optiplex computers are mainly for business users not gamers. Look at the XPS or Alienware models for gaming. Optiplex models are even sold on the business web site.

Dell T3500's are workstation computers yet they work fine for a gaming PC. A workstation is still a PC running Windows.