P7P55D Deluxe in need of memory.

The Asus P7P55D Deluxe can support up to 16 GB of DDR3 RAM. I'm not sure how much memory you really want/need, or how much of that $150 you want to spend, but here are a couple of options:

1.) Your motherboard supports 16 GB, so one option is to get 16 GB. You can go to newegg, tigerdirect, etc. and look for 16GB of DDR3 and find many options for under $150, depending on the frequency you desire. Right now there is a $20 off promotional code for 16GB of DDR3 1600 MHz (PC12800) at newegg.com for only $134.99 ($154.99 with the $20 off) with free shipping (http://www.newegg.com/Product/Product.aspx?Item=N82E168...). Not such a bad deal, but there are other options if you want to go with 16 GB, just have a look around.

2.) Just because your motherboard can support 16GB obviously doesn't mean you need 16GB. My motherboard support up to 32GB and I have done just fine with 8GB thus far (haven't breached the 6GB mark just yet). Most people do just fine with 8GB, it depends what your needs are. If you want to save some cash (8GB is nearly half the price of 16GB) then shop around for 8GB. If money isn't an issue then get 16GB. Even if you might never use it, more is always better then less when it comes to RAM. However, if you are not made of money, then only you can know how much RAM you really need.