Phenom II X6 1100T will not benefit from GTX 980. I had one and it got replaced by Vishera in AM3+ board. Vishera FX-8370 is about 20% faster than Phenom II X6 with broken turbo. If you do not fix the broken turbo it is only very slightly faster than Phenom II X4 955.
During my testing of AM2+, I identified GTX 670/770 as more than enough for Phenom II X4 955 to play Windows Vista/7 era games at max details in 1920x1080. If your focus is just Windows XP, you will be fine with GTX 660/760. I wouldn't really bother with GTX 980 and fixing driver. GTX 660/760 are so numerous and cheap that people give them out for peanuts. You can even request lower price for fun. If you find GTX 670/770 for the same price then go for that. I'm using GTX 780 in AM2+ only because I already have it as I needed it for benchmarking. I also have another GTX 980 (non Ti and Ti) collecting dust in drawer because it isn't necessary.
I'm not suggesting older cards, because for Phenom II X6 1100T it makes sense to eventually dual boot Windows Vista/7 as well. Phenom II X4 955 is not powerful enough to play games with Physics, Phenom II X6 1100T will have the same problem. There is major drop in FPS that makes it not worth it. You will need Intel 3770K for that.
GTX 980 really deserves Intel 3770K or at least very fast Vishera. I have also FX-9370 but it isn't installed yet until I determine how it compares with FX-8370 TDP wise at low frequencies as I would undervolt it and under clock it.
Pentium III 900E,ECS P6BXT-A+,384MB,GeForce FX 5600, Voodoo 2,Yamaha SM718
Athlon 64 3400+,Gigabyte GA-K8NE,2GB,GeForce GTX 275,Audigy 2 ZS
Phenom II X4 955,Gigabyte GA-MA770-UD3,8GB,GeForce GTX 780
Vishera FX-8370,Asus 990FX,32GB,GeForce GTX 980 Ti