If you want to buy a PC for making music on and are not limited by budget and cant be arsed building one you can do a lot worse than carilon.
You will need a decent MIDI interface if you want to use your keyboard. Also you could get a hardware drum machine, but personally i'd use software. Much easier and more manipulatable.
As for synths, a decent hardware synth is much more fun that fiddling with mice buttons... ;)
As for "Whats hot and whats not.."
NOT - Anything prebuilt from a major manufacturer. Steer clear, they often use substandard components and customer service tends to be on the crap side. (IMHO of course)
HOT - I cant tell you, that would take a LONG crawl through review websites. But here are some specs you might want to consider...
CPU.. not less than 3ghz
FSB not less than 533mhz
RAM - Not less than 2gb
HardDrive - Not less than 120gb and MIN spec of 7200rpm SATA not PATA.
MOBO (motherboard) Try ASUS, GIGABYTE
Connectivity - FIREWIRE, USB2 minimum.
Monitor - TFT only... you're gonna spend a long time in front of it, make sure its good
Mouse - Optical only.
Keyboard - test some out, make sure it will last and is comfortable, again you'll spend a long time in front of it.
PC case... Lan Li do some great ones
DVD burner, minimum.
Im sure others will add to this, im no expert and there is certainly, AS EVER a question regarding Pentium Vs AMD.
#M~y only other comment is... get a legit copy of windows... and DONT PUT YOUR AUDIO MACHINE ON THE INTERNET....