I'm running an ASUS Mobo with the 1Gb kit of this: http://www.ocztechnology.com/products/memory/ocz_ddr2_pc2_5400_gold_dual_channel-eol 

 I'd like to buy this RAM: OCZ Gold Series 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 667 (PC2 5400)

OCZ's link: http://www.ocztechnology.com/products/memory/ocz_ddr2_pc2_5400_gold_gx_xtc_dual_channel 

Will the 3Gb total play well together? It seems to be 4-4-4-12

 Any thoughts? (It took me a bit of tweaking of voltage to get the 1Gb kit working well. But now it runs like a champ.)

Mixing ram is generally not a good idea. Some people have issues and some don't. Whether you will or not can't be factored really, you'll just have to give it a go and see. Keep in mind though, that when you pair faster memory with slower memory, they automatically clock to the lower memory speed of your modules in terms of timings/ram speed. So if they don't end up being incompattible modules, you might want to run memtest which you can get forom http://www.memtest.org/

Mess with the timings and voltages and see what you can get out of it. If you have the patience that is to fine tune them for optimum timings/performance. Keep in mind though, there is a chance that the memory will conflict...But when it comes to mixing modules, it's hard to find out if it will work or not, since not many people attempt to mix memory. 

at least its low latancy