E.g. I selected USA http://usa.apacer.com/us/products/m...
That link and all links after this are on the Apacer USA web site.
If you didn't choose USA, how you find the modules for your mboard is probably the same or similar.
Select Products tab - Dram modules - Desktop memory
On the left, select Memory Search
That goes to here:
Scroll down to Search by Model Name - enter p5gc-mx/1333, click on search.
That takes you to the list of Apacer modules compatible with you model here, after you click on p5gc-mx/1333 on the resulting page:
See the Apacer part number at left.
Compare that to the part number on the module you already have.
If the part number for the module you already have is NOT on the list, Apacer is not listing it as compatible with your mboard. In that case, there's no guarantee the module you have will work along with the same part number module in this mboard, or any other module of any manufacturer's that is listed as compatible with your mboard model - it might, it might NOT.
You don't necessarily have to buy the module directly from Apacer.
You can search on the web using the part number for the module and buy it from any place that lists it.
"will it support dual channel with my current memory?"
Usually you have to buy modules that support dual channel mode in matched pairs of the same size, manufacturer, and part number.
More than one pair can be a pair made by one manufacturer, another pair, or pairs, made by another manufacturer - the modules don't all have to be the same size, but they must be the same size for a particular pair.
All the ram must be running in dual channel mode and be installed in the right slots, in order for it to run in dual channel mode - odd numbers of modules (1, 3, 5) will all run in single channel mode.
The bios runs all the modules at the specs of the slowest specs of all the modules installed if they are different. All modules should have the same ram voltage rating.
You certainly have to install the pair in certain slots on the mboard in order for it to be able to run in dual channel mode - consult the Asus mboard manual for your model - if you have the CD that came with the mboard the manual is prbably on that - if you don't, download it from the Asus web site.
I don't know if the modules will run in dual channel mode even if they are the same part number when they were bought individually.
In any case, the difference between a pair of modules each running in single channel mode and two modules of the same size running in dual channel mode is very tiny - at most maybe e.g. 5% better ram performance in dual channel mode - most people can't tell the difference with the vast majority of programs they use.
Adding more ram, if you have programs that benefit from having more ram available and they run noticably better with more ram, makes much more of a noticable difference than whether they are each running in single channel mode or a pair or multiple pairs is/are running in dual channel mode.