Honestly, a nice SSD would probably be about the same cost as the RAM upgrade, but will bring a much more noticeable performance increase.
If you're worried about too little hard drive space, you might want to look into something like the Optibay. It lets you replace the DVD drive with a regular 2.5'' SATA hard drive.
My Macbook Pro i7 has an 80 GB SSD and its original 500 GB hard drive in the optibay. I keep my home folder on the 500 GB drive and all of my apps (and the OS) on the SSD. It's a great combination of speed and storage space.
Here's a pretty good price on the SSD ($179):
http://www.newegg.com/Product/Produ...167031&cm_re=intel_ssd-_-20-167-031-_-Product
I can't remember the brand of the optibay-type device I got, but it cost me about $40. No need to spend more for the Optibay brand, if you can find a good alternative.
Admittedly, this isn't for everyone, especially those that aren't really comfortable working inside their computers. For your needs, 8 GB of RAM probably wouldn't make a huge difference. I have 4 GB and I almost never run up agains the limit, even though I'm a pretty heavy user - CS5, XCode, Safari tabs, iTunes, etc. Even if I start to use a bit of the swap, the SSD is so fast that I don't really notice.
Anyway, it's your money. Do what you feel is best.