Need advice on PC for web design-video editing $500-$800

Not much time for games anymore so I really just need a new box for working on websites and editing videos.
I do a lot of heavy multi-tasking with several programs open at once on multiple monitors.
Sometimes I'll find myself running photoshop, fireworks, dreamweaver, flash, excel, notepad++, a mail client and 20-30 open tabs in IE, FF, and Chrome at the same time... yeah, I have bad habits and need a PC that doesn't mind dealing with them.

I'm just starting to get into video editing, so I'll probably be mixing vegas, premiere and aftereffects into my workload more so I'd like to find something that can deal with that as well.

A friend suggested an HP that's on sale at compusa this week for $549.
HP 3130 LA056UT Desktop PC - Intel Core i5-650 3.2GHz, 4GB DDR3, 320GB HDD, DVDRW, Windows 7 Professional 64-bit
If I get this one, it has two RAM slots open so I would buy 4 more GB DDR3 for $40 bringing it up to 8GB total, and I already have an extra 1TB internal HDD, and a 2TB external HDD that I could add so I'm not worried about the 320GB HDD that comes with it.

What do you all think about this box?
Pros, cons?
Do you have a better off-the-shelf suggestion within my budget?
Does AMD have a better CPU in my price range than this i5?
If this looks like a good deal to you, is there anything you would add for my purposes?

Oh yeah, I should mention I really don't have the time or inclination to build my own (as nice as that would be) so I'm really just looking for something I can buy ready-made, load my programs, and be working the same night.
Thanks in advance for any help or advice.
Cheers!