First thing, and sorry, I gotta ask, can you change out the new patriot ram for the gskill? Second thing, mixing ram modules usually isn't a good idea but you don't know if you don't try right? So, Put it in and first run all 4 sticks at the stock voltage and speed settings. Both should run at 1.5v and 1333 speed. Check that first just to see if they will work together. Then, if that works, and after you have done that go ahead and put it in xmp mode and see what happens to the dram v. If it goes to 1.65v like I suspect go ahead and see if it boots. Most likely it will since you mentioned the timings were the same, but how about the command rate, cycle time and refresh rate? If it doesn't then you can keep them at the 1333 setting. You will notice the difference in the amount of memory more than the difference in speed. The only differences in running the 1.5v at 1.65v will be a heat increase and of course, shorten the MTBF, (lifespan), to some degree. Make sure you have good airflow through the case to help with the heat. But the best thing would be to get one of the memory coolers. Or this one.
Thanks for the reply. I was able to add the new ram in addition to the old ram and boot. Asus' "Auto" setting for the DRAM seems to work well; hwinfo32 reports the settings correctly, including the different states (XMP).