8GB of RAM is excessive on a PC unless you're getting a 64 bit system with multiple processors. Depending on what OS you're running, the machine only "sees" a certain amount of RAM anyway, and the rest gets ignored.
For example. My HP is not quite 2 years old, and when I got it it was one of their higher end models. I only have 2GB of RAM, and a 250GB HD (I use EHD's, so that wasn't a huge concern for me) but I routinely run PSCS2, Lightroom, and Firefox at the same time (with games and stuff going on in FF) and have no problems.
Enough RAM is certainly an important feature, but I wouldn't go as high as 8GB. 4GB is plenty (and you can always add more later on if it becomes necessary). Get a really good graphics card. Get a good size HD to serve your needs. Size will depend on how much you plan on being mobile with it (I love my 17" screen, but it's kind of a PITA to travel with, and it weighs a ton).