It's $15 extra for 8Gb over 4GB ....if 64 bit try and squeeze it in of possible.http://www.tomshardware.com/reviews/ram-memory-upgrade,2778-6.html
In a 64-bit system with a powerful graphics card, 8 GB of RAM really is a must if you're going to play demanding games. This is the case regardless of whether you're using 32-bit or 64-bit applications. However, even more than 8 GB of RAM can be subjectively noticed while playing. There is no real need for 16 GB of RAM, though. Going with 8 GB is quite enough unless you're running some taxing application in the background. We’ll comment more on this subject later, because the problem is often overestimated, as shown in our 64-bit gaming benchmarks.
Very few games show a drastic increase at x16 x16 today (typically 2-3%), but some do (i.e STALKER @ 10%) .... most review sites predict more will in the future. The price difference between boards that can do x16 x16 and those that can not varies substantially from week to week....but has usually been about $20
Right now for example, the NF200 equipped ES Revolution costs $225w/ rebatehttp://www.newegg.com/Product/Product.aspx?Item=N82E16813131714
The equivalent board w/o the NF200 is $215http://www.newegg.com/Product/Product.aspx?Item=N82E16813131701
For $10, it's a no brainer..... drop the rebate and not so much.