DDR2 upgrade, more MB or better timings?

I've been perfectly satisfied (so far) with my Kingston 2x512MB DDR2 533 running in dual channel. Apps are basic MS Office stuff. Games are Star Wars Battlefront, UT2004, MOH, and older NFS games. Also, digital photo viewing/editing (nothing intense).

Near future use will be BF2, newest driving games, graphics card upgrade to 7900GT.

I was thinking of upgrading RAM by buying a Kingston 2x1GB DDR2 533 kit with the same timings as the 512MB sticks and populating the remaining two DIMM slots (dual channel) for a total of 3GB (overkill, I know). If I didn't like the performance of having all four DIMM slots populated, I figure I can pull the 512MB sticks. But...

Would I be better off abandoning my current 512MB sticks now and just start looking for a higher end 2x1GB kit in DDR2 533 with better timings from the get-go? Would my MOBO even be able to use these "factory overclocked" RAM I've heard about? I have about zero overclocking potential (and desire) with my current equipment.

My rig:
Intel 915PBL MOBO with 3.2gig prescott
2x512MB DDR2 533 CL=4
XFX 6800GT (plan to upgrade to 7900GT)
Viewsonic 1200x1600
No RAID

Oh, and an Antec 450W Smartpower PSU