RAM:
I would recommend 4 GB of RAM (2 sticks of 2 GB each). You may not need that much, but RAM is pretty cheap so it's probably good to be safe. I forget what the pins do (looking some up, they're listed as "240-pin DDR3 SDRAM 4 GB" and things like that), but I do know you have to make sure that whatever motherboard you get supports that type. Any more than 4 GB is completely useless unless you know you're going to use 64-bit Windows... and tbh even if you are using a 64-bit operating system having any more than 4 GB won't help that much.
Processors:
I'm kind of out of touch with the newest lines of processors... but I've heard that Intel's i5 and i7 ones are extremely nice. Probably pretty much anything being sold right now will work fine... but I don't know the specifics anymore.
I do know that Intel chips tend to be more expensive than AMD ones, because AMD has a company policy that they don't advertise at all. The main downside to AMDs are that they run hot. Since you're using a desktop, you might not care... but on laptops the heat can be kind of uncomfortable. I've also heard that since they run hot, the processor wears out faster than Intel's do... but I'm not sure how true that is. My family's had the same AMD PC for about 6 years now, and it's still working fine. Well... the software is all out of date and it's slow because it's old, but nothing on it is broken yet.
Graphics cards:
I don't know too much about the specifics of these either, but I do agree you're going to want a pretty good one, if you're using photoshop and Vegas and stuff like that. The two brands you probably want to look into are NVIDIA and ATI. It doesn't sound like you're planning on doing anything with linux, judging by the software you mnetioned... but if you are, you should know that ATI has pretty shitty linux support. It works alright... but the 3D support is pretty bad. The other thing to note is that each brand of graphics card is works better with one brand of processor. If you're using an Intel processor, NVIDIA graphics cards tend to work better with it. If you're using an AMD processor, ATI cards work better.
Motherboards:
I left this 'til the end because you pretty much just want a motherboard that supports the other components you want. If you plan on upgrading your machine piece-by-piece (rather than just getting a whole new computer all at once), you might want one that supports a bit more than what you want, so there's room to upgrade... but it just depends on what you want. Each Motherboard will list how much RAM and what type of processor it's compatible with. You'll see something like "4