ZX, a 250W power supply is going to be too small. It might work, but I wouldn't risk it. (Certainly not if you plan on getting a different video card at some point.) I'd get a 300 - 400W range power supply. If you've got the money, a 330W or 380W Antec TruePower power supply is top of the line.
The A7N8X-VM is a micro ATX board, meaning it only has 3 PCI slots. I've used it once now and it seems ok, but it has absolutely no overclocking or tweaking options in the bios. It would be a good board for an office machine or a basic home pc. I wouldn't recommend this board for an extreme gaming rig or for someone who likes to tweak with their PC. It does NOT have the full nVidia Soundstorm audio either. (It does have onboard sound that is decent however.)
There are 4 nForce 2 chipset based Asus boards that I know of:
A7N8X Deluxe - Top of the line, with the full nVidia Soundstorm audio plus lots of other extras. (Firewire, Dual network, Serial ATA, etc) This board DOES NOT have integrated video, only the -VM has this.
A7N8X - This board lacks the Soundstorm audio of the Deluxe model, but still has adequate onboard audio. It also lacks some of the other goodies the Deluxe model has.
A7N8X-X - This is a budget board. There's not many extras with this board at all. Basically you get a floppy ribbon cable and a single IDE drive cable and that's it.
A7N8X-VM - As noted above, this board is a MicroATX board with onboard GeForce 4 MX video.
I personally prefer the Deluxe board out of these 4 if you have the funds to afford it. (Remember you will need to buy a video card since it doesn't have onboard video.)
As far as the MSI boards, I haven't personally built a system with one of their nForce 2 boards. They look OK on paper, but I can tell you that any nForce 2 board with integrated video will not support the 3200+, only up to the 3000+. (That comes straight from nVidia, only the non integrated video nForce 2 chipsets officially support the 400 MHz FSB that the 3200+ requires.) This isn't really a huge loss, considering the minor performance difference between the 3200+ and the 3000+.