Sorry, in THAT case it won't support it.
The Dual-775Vista has 4 memory slots, 2 are DDR1, 2 are DDR2. You can only use one type at a time, though. It also sports both a AGP port and a PCIe x16 port (really running at x4 speed but I haven't had ANY performance problems so far), so you can pass your older stuff onto a newer motherboard.
SATA is soooooo much better than IDE. It's wicked fast compared to it, as well. The RPMs may be the same but the drive/interface combo is way faster. Read/write times are much faster. If you must, get a smaller SATA and install Windows and program files on it, then all your extra storage on the older IDE (just don't swap to and from the IDE while in a program if you can help it).
I have a 475W PSU that is of good quality and reputation. It cost me about $60. The one I had before it was listed as 350W but was a total POS. You can actualy fry your computer by undervolting or overvolting, and my PSU was doing BOTH very badly (on different areas). Some of my hardware wasn't working, and the other stuff that was was in danger of being killed.
The PSU is more important than the CPU. Keep that in mind! (also you can take the PSU with you to other cases in future upgrades).
What motherboard are you considering so far? Keep in mind if you're bringing your old cards (NIC, sound, fireware, whatever) you will need to get a motherboard that still has PCI slots. Many only have 1 or 2 nowadays, and several PCIe X1 slots (which are useless at this point in time). For conroe chips you'll need 1066MHz FSB, and even the 6320 (only 1.8GHz) will blow away most older chips. The performance gets better with higher conroe versions, but it's a good starting spot (and cheap!)
EDIT: Humble was typing the same time as I was! He's got some nice choices there!