Windows 7: Upgrading computer memory deciding help.

I currently run window 8, and I only have 2gb of ram. Im running cosair 1333mhz 1.5v 2slots x 1gb ram each.
But my question is can I run a third slot of 2gbx1slot and if I can will it be slowed because im using an odd amount? I only need around 4gb and I feel that running 6gb is a waste of money.
my motherboard is the intel dx38bt, im a bit confused because it lablels the slot like
1xa (memory slots)
1xb
2xa
2xb

and my ram sticks are in (1xa) and (2xa), and if I do buy 1 slot, would i just stick it in any left port?



links.
Newegg.com - CORSAIR XMS3 2GB (2 x 1GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Dual Channel Kit Desktop Memory Model TWIN3X2048-1333C9

It will be absolutly fine. Aslong as you put the 2gb stick in 1b. And keep the 2 x 1gb sticks I'm 1a and 2a

Will it be bad because I only have 1 stick, seems uneven lol.

That's why the dimms are in pairs . It will be 100 percent fine. Go for the upgrade.