|"I think my big problem in the slim tower"|
Why haven't you posted the make/model of your system?
Since you have a slim tower, you're screwed. A standard "off the shelf" power supply probably won't fit, plus the choices in low profile gaming cards is fairly slim (no pun intended).
low profile gaming cards at newegg
The 1st card on the list is a lower end gaming card (but it IS a gaming card) & will probably work with your exisitng power supply.