lol nice double post ;o. with a 500-600 dollar budget, ur probably not gonna be looking at something crazy. but lets break it down for u and show u ur options (btw, im getting all of these items from newegg as well, so u should be able to find them there).
processor: the AMD X2 processors are definately nice, but if ur lookin to spend 500-600 for a complete system, those processors r going to be too expensive. a single core
AMD Athlon 64 3700+ or 3800+ should be fine for recording (i use a 3200+ and i havent seen any problems yet with 30+ tracks on 1 song).
motherboard: depending on how many extra things u want with ur motherboard (like firewire, how many pci slots, pci-e or agp, onboard video, etc.) u have a LOT of choices. u dont need anything with SLI support and the good brands out there are Asus, Abit, DFI, MSI, and i personally like BioStar. other brands r pretty hit or miss ive noticed. my personal recommendation for a budget computer would be a BIOSTAR TForce4SLI.
memory: however much ram is all up to u. u have 4 channels to work with (2 dual channel setups). u can either get 2 1gig chips or get 4 512mb chips. dual channel setups generally are going to work better (biggest implementation of the AMD 939 pin processors). i only have 1 gig on my computer and it works great. so yeah.
videocard: if u decide to go with a board without onboard video (like the TForce4SLI), then just invest in a cheap videocard that will get u a good resolution on ur screen (almost anything will work, just make sure its compatiable with ur motherboard)
hard drive: depends on if u wanna go SATA or PATA. PATA is the old drives with the huge cables, sata being the new generation with very skinny wires. (WARNING: with a lot of sata drives, u need to have the drivers when installing windows on the hd). also depends on how much space, but other than that its all up to u. around 200gigs is where u get the best price for amount of gb, but u dont have to spend that much if u dont think ull use it.
optical drives: if u need it, on newegg theres the NEC 3550a which is a DVD-RW/CD-RW drive for like ~$40. i love it ;o
case, powersupply, and anything extra like that: just look around. some r better than others and u just gotta do ur research.
other than that, ull probably need some form of an audio interface (i dunno if u have one already), and if u need suggestions on those theres PLENTY of threads in here to help u on that.
enjoy

and if u have any other questions ill try to look back asap