First, there are several bottlenecks at play here and upgrading memory is the least of your concerns. Your processor and mainboard chipset will bottleneck no matter how much memory you put in your system (or even installing Windows 7).
I recommend upgrading your computer all together and getting at least 8GB of memory. If you're looking to save money, AMD still has great performance to value ratio. I myself have an AMD FX 8350 and 990FX mainboard with 16GB of memory. I haven't noticed any hiccups when editing in photoshop, premiere or after effects. If you're looking for power then Intel CPU's will be a good place to start.
Are you looking to save money or looking for more power?