Right, then AMD it is, which you've already decided
These will be better for games than Intel, oh and if you want to multitask games... you'd have to have a top system running on SLI with two video cards adn possibly a dual cpu too to play both games
Anyway, your budget is $550 and you could get some good parts with this and luckily AMD is cheaper to go with than Intel, so lets check.
First of all, a great AMD Athlon 64 3000+ would do you nicely (runs at 2.8ghz in AMD terms, but would be just as good as a 3.0ghz Intel Pentium 4) running on a 939 socket Venice core, and so needing a 939 socket motherboard.
Look here for a bit on it:
This would cost you $146 from here if you could
Next the memory, and 512mb should be enough for now, but because of the motherboard choice being open to you, the memory speed the RAM speed you will have to buy will of course differ, though most mainstream ones use PC3200 speed RAM or in other words DDR400.
Now the Video card. I'd personally go for Nvidia since the cards support the latest Direct X version for better effects (9c, compared with ATI's version 9? I think). Anyhow, I just like them better
Though, I have an ATI card and so not biased by the way...
A good Nvidia card would be a 6600GT running on a PCI-Express motherboard (so take note of that since all new cards will support this in the future):
Heres an example of a PCI-E version:
This would cost you $169.00 and would play all the latest games in a standard resolution (lower than 1600x1200) and at high settings
So far, without the motherboard and memory, there is a total of $315. So I'm sure with the remaining $235 you can get some great memory, and a good motherboard, since this would really be up to personal choice, i.e do you want onboard sound, firewire for connecting dv cameras, etc.
I hope this helps
EDIT: If you can afford a standard Nvidia 6800 after making the motherboard and memory disicions costing about $256 then go for it, as it would power for longer of course than the 6600GT would:
I tried looking for a 6800GT, but I know for sure that would be out of your budget range without really getting bad memory and motherboard.