Brand shouldn't really matter that much, it's more about the hardware. Sure, one brand may have better tech support than another...but unless your problem is something simple like "how do I restart my computer" you'll probably end up equally frustrated with any tech support.
You don't need a dual-core processor, but editing is one of the applications of a computer that can really make use of multi-core processors. So I'd suggest getting a dual-core (or perhaps tri-core if the price is right). Quad-core or above is overkill if you just edit the occasional video.
The amount of ram you should get depends on what else you want to do, and the OS you get.
Stuff like editing takes a bit of ram...but it doesn't require a ton of ram like motion graphics or 3D modeling/rendering would. Also, 64-bit OSes have some higher overhead compared to 32-bit ones...and 32-bit OSes can't use more than 4GB of ram.
That said, if you're looking at XP or 32-bit Win 7 I'd suggest 3GB
If you're looking at 64-bit Win 7 I'd suggest 4-6GB
There's little sense in getting the max amount of ram you can get if you aren't going to use it, and most people won't use even 4GB (I don't think I've even used more than 3GB on win7 64-bit or XP pro, even with multiple programs running and around 50 tabs open in FF)
Video cards don't really come into play for editing, so any modern video card should do the trick unless you plan to play stuff like crysis as well.
I don't know what type of videos you do on youtube, but if you record gameplay you may want a capture card. Unless you went to a computer boutique dealer to get a computer customized you'd probably have to buy and install the capture card separately. I don't have experience with capture cards though, so I can't really give concrete advice. However, I'd lean towards an internal capture card (like the Hauppauge Colossus
) over something like a Dazzle because of the data rates involved (especially for HD stuff)
Adding hard drives is a pretty simple affair, so don't sweat the storage thing too much. I'd go for traditional HDDs over SSDs for you since you want lots of storage. Stick with 7200rpm drives and you should be fine. I believe most brand name computers are using SATA drives now (as opposed to the older IDE/ATA/PATA drives), just one extra little thing to keep in mind when shopping for drives.
Some pre-built computers that should get the job done fine:HP Pavilion p6770tDell Inspiron 570
(pretty much anything not the economy variant)Acer AM3400-U2502
If you're feeling adventurous, you might give building your own computer a shot.