That Gigabyte MOBO has dual channel memory slots.
You can populate and run with one 4 GB stick and add a second as funds / needs dictate.
Current Microcenter circular has a PNY (name brand) 4 GB stick of DDR3 on sale for $37.00, a no brand 4 GB stick for $34.99 and a Kingston 8 GB 2 stick kit for $79.99
Or Tigerdirect, 1 4GB stick DDR3 $35 out the door plus $5.00 rebate get's it to $29.99. Disclaimer: tigerdirect were a partner when I was at CA and their rebate center sucked, you need to do everything right.
ADATA XPG V1.0 Destktop Memory - DDR3 1600, 4GB, Blue - AX3U1600W4G11-RD at TigerDirect.com