Something to think about considering how questionable you system is. Even with the best CPU open to Socket 775 which you may not be able to use with that motherboard, your not going to make full use of that card. You would also have to replace your power supply to make use of it.
MSI H81M-P33 Micro ATX LGA1150 Motherboard $48.99
http://www.mwave.com/mwave/skusearch.asp?px=CJ&scriteria=BA78100#.VD1hIxYbKbU
Pentium Processor G3258 $69.99
http://www.amazon.com/dp/B00KPRWAZQ/?tag=pcpapi-20
G.Skill Ripjaws Series 8GB $72
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231314&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
EVGA 600B Bronze 600W 80 Plus Bronze Power Supply $39.99
http://www.ncixus.com/products/?usaffiliateid=1000031504&sku=89067&vpn=100-B1-0600-KR&manufacture=eVGA&promoid=1081
EVGA GeForce GTX 750TI 2GB GDDR5 128BIT $115
http://www.ncixus.com/products/?usaffiliateid=1000031504&sku=94331&vpn=02G-P4-3751-KR&manufacture=eVGA&promoid=1444
$345
Blows what you have out of the water in gaming
Gives you potential with the larger power supply
Gives you great CPU upgrade paths
Open to all current GPUS
Is about $140 ish more than you were planning on paying just for that R9 280
For about $40 more you can swap out the Motherboard for a Z87 and overclock that Pentium up to 4.4ghz with a aftermarket cooler.