I'm not so familiar with pre-built computers, but hp/compaq has done well for me in the past.
Sony's Vaio looks to be among the better pre-built (though I have heard rumors about the vaios coming with a lot of bloatware)
If you're a little adventurous, you can try building your own pc (and you might save a little cash that way too); but if you're not, you can still use this list as a guideline. This list should be somewhere in between budget pre-built and ultra-high end custom (closer to ultra-high end though)
processor:
Intel Q6600ram:
some corsair ddr2 800 that's on sale, 4GBcpu cooler:
Arctic Cooling Freezer7 propower supply:
Corsair 650Wmobo:
gigabyte GA-EP45-UD3Rmonitor:
Acer 22" WidescreenHard Drive:
1TB WD caviarVideo card:
HIS HD4830DVD Drive:
LG dvd burnerSpeakers:
Logitech Z-4total cost after rebates and shipping: $981.85
I didn't put an OS in there, so add another $135 or something
Quad core processor will help with encoding (more and more editing programs are able to take advantage of multi-core processors as well)
4GB ram should be more than enough, I only use all of my 2GB when messing around in AE anyway
1TB should be plenty of space, but they're [relatively] cheap if you need more
That's it for the core essentials, everything else is just icing on the cake
Video card lets you play games, and some (read: very few) avisynth filters can use the gpu to speed things up (I <3 fft3dgpu() )
Power supply gives you plenty of room to upgrade in the future
CPU cooler makes sure your cpu stays cooler (which makes the cpu last longer)
dvd drive is a given, if you are planning to just scrap your current computer I'd salvage the keyboard, mouse, monitor, speakers, dvd/cd/floppy/misc drives and see if you can reuse your HDD with the OS you have now
if you do go with a pre-built computer here are some things to keep in mind:
you want discrete graphics, not integrated. Integrated graphics (with a few rare exceptions) claim some of your system's ram (so if you had 2GB and integrated graphics you might only see 1.75GB of system ram.) If you do want to play any relatively new game at decent settings you'll really want a mid-range graphics card.
SATA Hard drives are better than EIDE/IDE (ATA) hard drives. You [probably] won't notice any difference until you try to move a 1GB project from your computer onto an external HD, but depending on what motherboard you get you may only be able to use 2 ATA devices (the ATA interface is commonly used by DVD/CD drives and HDDs)
Processor speed doesn't really matter as much as the type of processor. Higher is better, but it's not usually worth the extra $$$ (if you're lucky you might be able to overclock a little to get a little extra processing power)
Norton sucks, don't let the sales guy at best buy talk you into it. In fact.?.I wouldn't buy any of the security suites/software you'll find. Avast! antivirus (free) + zone alarm firewall (free) + spybot search + destroy (free anti-spyware) has served me well for the past 3 years (and free too!)
If you must buy a security suite, I'd go Kaspersky
Vista = meh, XP still works.?.I'd "Downgrade" if it was available