First of all, let me lead you here which might help with some components:
Now, do you want to go AMD, or Intel for your processor? AMD is better for games, whereas Intel tends to exceed better in multitasking and video editing, though AMD are also good at these.
When you choose, we could give you choices and tell you more about the chips available. As you seem to want to do video editing and games, I would suggest a Pentium 4 processor, with something like HT technology (basically HT means Hyperthreading, and again gives the processor a boost in things like video editing, acting like it has two CPU cores when it only has one)
Now AMD mentioned in the link of the above post uses a new technology in teh new chips, which comes in the form of the pin 939 chips (the fastest ones). These will probably be used for a while for AMD if you chose them and so updating wouldn't be a problem for a processor either. If the chip suddenly changed though, it would mean if you wanted to update, you'd have to swap motherboards too, though you shouldn't need to do that in a very long while, by which case, you'd probably want to build another
Pentiums, I'm not really sure how they update, though they use the same type of pin architecture too.
As for the video card, there are two types used now, AGP and PCI-E. PCI-e is the newest form and started out from a slot called the PCI port, used in all systems to add components to add functionality, and before the AGP slot became known, PCI was also used for video cards, though the limit had soon been reached and AGP became standard.
Now, the same is basically happening with AGP, with PCI-e offering faster bus speeds ( how fast data will travel to and from the card) and more power to cards.
For a new system, I would honestly go for PCI-e since alot of cards are available, and if you wanted an AGP chip (since motherboards only contain one of the slots), there is something called a bridge chip which will cross between the two. It will also give you a better chance to upgrade later on.
Now you will probably already know that two video card manufacturers are always battling out to deliver faster cards than the other, Nvidia and ATI.
They are both about the same in games to tell the truth, though they all use different architecture in their cards, though this doesn't affect much. As for all video cards, all the new ones support the new Direct-X, Direct-X 9 which offers better effects in games.
Now soon, new bunches of video cards are being released by Nvidia, which will of course be closely followed by ATI. but luckily, video cards are simple to update!
Most motherboards by the way have an onboard sound chip, though I'm sure any external card is better than one of them! so you'll be fine.
For memory, 512mb is becoming the normal, though many people also choose 1gig which would give them better chances to run more programs at once, and just give the system a little boost.
I'm sure we can find the right system for you
I hope I have given this a kick start