I think the g3258 is, but it isn't that great. An i3 would probably be better. The next unlocked CPU after that is an i5 4670k/90k, which is a LOT more expensive than the g3258
An overclocked g3258 > i3; g3258 not overclocked < i3. I would recommend something with 4 cores though I don't think 2 will cut it for that much longer AAA titles.
The G3258, is an outstanding cpu if you will overclock it.
The nice thing is, that the lga1150 motherboard will allow you a future upgrade to a i3,i5, and i7.
If you pick a H97 or Z97 motherboard you will be compatible with upcoming 14nm broadwell processors.
While it is possible to build something for $450, you have to cut too many corners for future growth. The OP wants to upgrade later, so, to me, that means this system must have the final motherboard, case, and PSU.
Here's as cheap as I would start. No monitor, because you are going to get a second hand pawn-shop one or second hand flat-screen TV and upgrade it later.
The CPU can be upgraded to an i7 4790K and the PSU will run it overclocked, and you can add a GTX 970 GPU if you want to. Memory can be expanded too. It will run on 1 x 4.