A Radeon 4850 huh? That's a nice card. What kind of PSU, HDD, and case did you get?
Sadly there is no way you're going to get an i5, a motherboard, and DDR3 for $300. A good idea when searching is to look at Newegg's Combo area, where if you buy two items together you get a discount. Check around the cheapest i5 processor's combo deals, see what you can get for $300:
http://www.newegg.com/Product/Produc...Size=10&page=1
As you can see you usually end up paying more than $300 for an i5 and motherboard, not even including RAM. You
can get CPU+mobo for very slightly under $300 combined, but that means it's a low quality motherboard. A cheapo ECS and the like. And if you have an i5 and a Radeon 4850... you will not want to skimp on the motherboard lol. Not to mention RAM.
Instead... I'd go with this, for your price range it's the best hardware you can get. Others, if you find better deals please correct me.
http://www.newegg.com/Product/Produc...Size=10&Page=2 and it comes in a combo here:
$214.99.
The Phenom II architecture is a bit less efficient per GHz and per core than the i5, but keep in mind there are
4 cores, as compared to the much more expensive i5 with
two cores, plus two virtual cores thanks to hyperthreading. Those virtual cores are nice, but won't do as much in games and some other areas as real cores will. An overclocked Athlon II will usually beat an i5 in gaming benchmarks, and Athlon II's are just Phenom II's without L3 cache. Phenom II fares better than Athlon II basically. The virtual cores don't help the i5 as much for gaming as physical cores do, although they are great for media encoding, unzipping, and synthetic benchmarks, if you like to brag about your 3DMark score lol.
Use the leftover money to get yourself some nice ram, a la...
http://www.newegg.com/Product/Produc...82E16820227483
We're a few dollars over $300. But if you try and squeeze in an i5, a motherboard for it, and RAM, first I mean you can't get those parts for that much money, and second even if you increase your budget to $350 or $375 (because you do need RAM), the motherboard will still be cheap crap...