I've always wondered what people do
on a MacBook Pro to fully utilise >16GB of memory in 2013? That's a genuine question, not trying to be sarcastic
As for the OP, 8GB will be more than enough to run those relatively lightweight programs. I use very similar tools (I'm a junior web/mobile app developer) on a 2010 27" iMac with 8GB RAM - I use Coda 2 over Dreamweaver, DW is horrible software! Of course, the other hardware in my machine will outperform yours, but purely from a memory usage perspective, it should perform nearly identically.
Memory is so cheap nowadays, it's almost worth you giving it a go...if it doesn't work out, you can look at other options without having lost much money at all.
Here's a good place to start looking, check your model number for the 16GB trick wonderspark mentioned:
http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3
That said, the best thing I ever did to my setup was put a SSD in my iMac. The performance jump was beyond anything I'd imagined.
Edit: With an external display plugged in, depending on resolution, your laptop will likely run hot. My 2009 15" really doesn't like using my 27" iMac as a display, especially not when using Photoshop. I'm waiting for the Haswell graphical performance/power usage improvements before replacing my MacBook for this very reason.