I'm guessing since I haven't gotten that many comments back on this that this is a good build, right?
No complaints from me. Just keep in mind that to get the best performance out of the Pentium G3258, you will want to overclock it, and that means you'll want to get an aftermarket cooler. It's $20 for the Cooler Master Hyper 212 Evo and it fits in the Antec 300. Well, if you want part suggestions.. the only ones I have will increase the cost of your PC.
The Asus Z97-E/USB3.1 motherboard is $75 after $40 rebate. Yes, it costs more money at first, but if you don't mind rebates, then for around $7 more than the cheapest MSI Z97 motherboard, you could get the cheapest Asus Z97 motherboard that comes with USB 3.1 ports. For about $10 more than the WD Blue 1TB, you could get a Toshiba 2TB drive. Also, the XFX R7 370 is available for $100.. but seems to be on par with the R9 270. Can't figure out which one is better for sure, they're nearly the same performance-wise.
Alternatively, you could pick up an i3 processor and a cheaper motherboard instead that wouldn't need an aftermarket cooler because it can't be overclocked.
PCPartPicker part list / Price breakdown by merchant
CPU: Intel Core i3-4160 3.6GHz Dual-Core Processor ($96.99 @ SuperBiiz)
Motherboard: Asus H81M-D PLUS Micro ATX LGA1150 Motherboard ($41.00 @ SuperBiiz)
Memory: Team Dark 8GB (1 x 8GB) DDR3-1600 Memory ($28.99 @ Newegg)
Storage: Toshiba 2TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ Newegg)
Video Card: XFX Radeon R7 370 2GB Double Dissipation Video Card ($99.99 @ Newegg)
Total: $326.96
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-11-28 02:24 EST-0500
Whether the Pentium or the i3 will perform better for games will depend on which games you are looking at. Some games will run just fine on two cores and benefit from higher clock speed from overclocking, while other games will run better on the i3's hyperthreading that makes it act like a quad core. It can be said that going with the Pentium build is a bit better if you are willing to overclock, and the Z97 motherboard would be a better match for a potential upgrade to an i5 4690K that would also be overclockable.. whereas the i3 build's cheaper motherboard wouldn't be very well suited to overclocking at all and be best paired with something like an i5 4460 or 4590.