If you want to upgrade only your mobo and cpu, and keeping your video card, then I would suggest to use a nforce2 board with a AMD barton cpu, either a mobile 2500+ which can be oc very well, or a higher rated barton CPU. Your video card is not top of the line, but sufficient. My point is, in some time, new technology will become available that will improve overall performance. New nforce3 250 chipset, socket 939 AMD64, pci-x...that would make today system .."obsolete. in term of technology. By spending only for the mobo/cpu/ram option, you will improve your gameplay, and your system will be balanced with your video card.
I'm in the same situation. I was thinking about upgrading, but with all that is coming, I just put a mobile 2500+ and run it @ 2.4GHz(1.8stock) and the performance is very adequate. My suggestion, since your from Canada, is to go to www.ncix.com, subsribe to their newsletter and you'll receive by email specials to upgrade.
I dont know Intel, so I can't recommand anything. But you'll likely to see the soltek 75-frn2-l nforce2(same as mine, except, I have the -rl) for around 90$ canadian, and good OCZ memory for dual channel and CPU. Maybe the mobile 2500+ will be price good too. I got mine for 119$can. You'll need a fan for, since it is OEM. Maybe you will be able to find a good PSU(350-400W) and stay within your money.
You'll find deal on Intel system too, But as I say, I cannot comment on them. But for now, I wont spend too much on high specs system...
-Always put the blame on you first, then on the hardware !!!