I don't know a good motherboard. Mine's only so-so (it does what I need but not the best). If you can hold off, I think they'll be releasing the successor to the Conroe sometime this Fall. It's got a higher FSB. Even if you don't go for it, it will lower the price of current chips even further.
If you can't wait, here's my top "budget" choice.
CPU:
E6420 (4MB cache) (~$186 on newegg)
or if you have more $,
E6600 (~$222 on newegg)
Motherboard is going to cost anywhere from $70 to $200. This isn't my area of expertise, as I haven't had a conroe chip for very long. It MUST have PCI Express (not AGP) otherwise you'll have to replace it at a later time, and waste money.
Vid Card: (ATI and Nvidia suggestions for price)
$100 range: X1650 or Ge7600GT (avoid 8500s and other 8000s below 8600)
$150 range: X1950 or Ge7900GS/Ge7950
$300 range: Ge8800GTS 320MB.
Personally I won't get an 8000 series unless it's an 8800GTS with 320MB or more. Anything less than 320MB is a stripped down card. X1950 seems best bang/buck right now, for upper-mid-range cards. Ge8000s aren't worth it IMO (and this is just my personal opinion based on what I've read).
PSU:
I don't know that many good PSUs... I know the one I have is from a good manufacturer. I did a newegg search (just one of many places you can look) and found they have 500W PSUs from $60-$90 range. Plan on spending $100. I have a bookmark from when I got mine.
http://www.devhardware.com/forums/power-supply-units-98/how-to-choose-a-power-supply-94217.htmlIt lists good PSU manufacturers, crappy ones (avoid those), and OK ones. It will help you in your search.
HD: If you can't use your existing one, plan on spending $150 for a good SATA hard drive.
CASE: If you want a new case, you need to add anywhere from $30 (my barebones case cost this much) to $120 or more. Average is probably $50-$60.
CPU: ~$200
MOBO: Probably around $100
VIDEO: Say $160
PSU: Say $100
RAM: DDR2 or DDR3 expect to spend $120 for 2x 1GB sticks
Assuming you keep your current monitor and CD-ROMs and all that stuff, this adds up to about $780 or so. If you already have DDR2 ram and get a new MOBO that accepts DDR2 you can save some money by using what you have.
All prices subject to change, this is only one combination of an infinite number of possibilities.