This is a $650 gaming and multitasking PC. This is my first time, and I've been doing a lot of poking around on Newegg. I'm not sure if I'm ready to buy, so is there something I'm missing or any way I can improve this build?
just changed the power supply up 100w and modular. For a first build and in general its nice to be able to add and remove power cables and its easier to route them for cable management.
everything else is good unless you change your mind on the i5.
I put an Overclockable CPU in there, just in-case you change your mind, you don't have to, but if you want extra performance, you have it.
Since this is gaming focused build, i mixed and matched different components. I managed to slip in an R9-290 into the build which is MILES stronger in performance than the R9-280 in your original build, and will be excellent for all your games on Ultra and even for 1440p later down the road if you so desire. there is nothing wrong with going AMD in a build in this price range, they make excellent CPU's for the money. also in your PCpartpicker link it says you purchased the case, so i removed it from the link to cause any confusion, to the price. but this is something i'd go with if i were in your position.
The build you posted is showing up with a 4 x 2 GB kit from Corsair for $103. I'm assuming that was not intentional? $103 for 8 GB of RAM is kinda really expensive.
Or even save up some cash and go this route. Either way, with the two cpu's you have overclocking potential, and will be able to run AAA titles pretty well.
Here's my take, drop the CPU a little bit to the FX-6300 and put more money into the GPU for an R9 290X. The 290X's have dropped drastically in price lately. Games are not CPU bound as much as they are GPU bound, even though people keep saying they're becoming "Multi-Core Optimized" its a bunch of BS. You need good single core performance for gaming. The i5 is better at that, but the FX series isn't going to leave you behind either.