I don't think there are any decent modern MBs with both AGP and DDR2; perhaps you could find a good quality used one on ebay. Overall, you'll save money by getting rid of your AGP card and moving to a current PCIExpress/DDR2 MB.
I'd just buy the various parts separately and assemble it yourself (or pay a local to do it). One site you didn't mention was Frys.com. They ship to Canada, and have various specials which are sometimes awesome deals -- you just have to be patient to accumulate the different parts you need.
I agree with Mondo, I'm not sure your stated combination even exists. You should consider a piece-by-piece upgrade that'll do what you need. Check this: ASRock-VSTA 775 mobo takes DDR and DDR2, and AGP and PCI-e. Consider getting that mobo and a CPU, you can use your DDR and X800gto for now until you can upgrade more later.
Seriously, it's probably your best bet. You can upgrade in phases, and have better control over what you want and when you can get it.
As for a case, I get cheap stuff that has a high wattage rating. I know it is not a good power supply, but it is good enough. NCIX has overpriced cases and power supplies, but you can get them anywhere. Just make sure the power supply is over 420 watts and you should be fine.
Memory is overpriced. If you wait for the summer, I am certain you will get a better price. I am sure you got some old ddr memory you can use to hold you over!
I suspect you can get other parts for cheaper when you live. I know my dealer does not stock ASrock boards, but he could special order it. I think it is better to deal local unless the price is really low on the mail order.
As for my preference for an AGP compatible board rather than a pci-e is because I've purchased an x800gto and haven't even been able to use it yet. I purchased it assuming that it would work with my set up as long as it was AGP -but I was wrong! Apparently my mobo (asrock p4i45gv) doesn't work with any ATI cards above the radeon 9800. I should have done more research when I built this comp, and when I bought that card...
Anyway, what I'm left with is a bit of a pickle... My budget is too limited to buy a new comp & good video card, so I have to either get a AGP compatible board, or go without a video card for a while (which would be a shame, having just spent $100 on one) - The two lesser sacrifices would be to build my own system, or to stick with DDR memory.
I've looked around at some of the suggested sites and am pleased so far, I'll keep at it and hope I find the computer for me.
I've decided to just make it myself, using the x800gto I've already got, the above suggested ASRock motherboard (already purchased, and being shipped), and am buying the Case/PSU/CPU/RAM at a local store called SLACO COMPUTER.
$276.80 is the total cost in Canadian dollars of the components purchased locally. The MotherBoard and Video card that I've purchased online totaled approximately $200. I'll be using 2 of my spare hard drives that have just been sitting around collecting dust.
$276.80 Canadian = $238.97 USD
A few upgrades I may do in the near future:
+1GB PC5400 DDR2 RAM
(bringing total to 2GB)
+$200-$300 PCI-E Video card
(If: I start using more demanding games or software than I do now)
Looks good. Just one thing to keep in mind: Depending what graphics card you end up going with, you NEED to ensure your power supply will be adequate. You may not be able to upgrade your card unless you also upgrade your PSU. So keep that in mind, very important. So then it depends on budget.
Also, make sure that mobo can take both DDR and DDR2 at the same time like you've shown. I know it can can DDR and DDR2, but check to make sure at the same time.