As Ramhound has already said, the 2GB is a hardware limit of the N550 Atom processor: http://ark.intel.com/products/50154/Intel-Atom-Processor-N550-1M-Cache-1_50-GHz
The best you can do is get the fastest DDR3 which the system will support. I can't find anything specific about the fastest you can use in the Asus EEE PC 1015PEM but I did find a few laptops using DDR3 1066 ram.
Newegg's memory finder lists modules up to DDR3 1333 (http://www.newegg.com/MemoryFinder/#y8wBAbp0ad0ABD) for all the included Asus Eee PC 1015* notebook's (up to 1666 for a few of them) but there is no listing for the 1015PEM. Of course the price difference is minimal for trusted brands like Kingston and there is no downside to using RAM which is too fast for the system, it will simply run at a slower speed.
Of course, the real limit is the processor and the demands of Win7. Try disabling all Aero fetures and any other "eye-candy" if you haven't already. If you're adventurous you may see noticeable gains from using a lightweight linux distro.