Oh, I thought since you posted the link you could buy it on eBay.
In that case, if you're planning to upgrade, I'd suggest either an ASUS P67/Z68 socket 1155 board (non-LX or LE versions for better overclocking as those have weak VRMs), or a Gigabyte Z68 socket 1155 board (expect to pay ~$140 or more for one of the better ones), along with a 2500K CPU. 2500K's are fast at stock, and overclock easily to 4.5ghz or higher with minimal cooling required (a cheap Cooler Master 212+ will be more than sufficient).
You could get an AMD system for around $100 or so less than the Intel build, but AMD's K10 CPUs are comparable to 2007-era Intel parts in clock per clock efficiency, and you'd be limited to 4ghz or lower overclocking with an AMD K10 chip in a 64-bit OS, which you should really be using at this point. If you haven't updated to Windows 7 you probably should.
Additionally, AMD's new architecture, Bulldozer, is handily beat by a 2500K even when heavily overclocking the BD chip (and keep in mind you can OC a 2500K up to 4.5ghz to 4.8ghz fairly trivially), and BD even loses out to
AMD's own K10 chips in many tasks. Also, BD chips require a massive amount of power to overclock compared to an Intel chip. BD's come in "8 core" variants, but due to architectural problems and inefficiency, Intel's 2100 dual-core Sandybridge is actually a better performer in most tasks:
I mean, this is kinda embarrassing honestly.
Other than for stability, replacing your current board/CPU with AMD equipment
won't actually be better than what you have now. In many cases it would actually be a downgrade. If you can budget it, I'd strongly suggest going with Intel as the extra cost would be worth it, given that you'd be able to use a 2500K for a longer period of time (with moderate overclocking) compared to the shorter usable lifespan of an AMD chip.
Additionally, AMD is largely pulling out of the desktop CPU market, so you won't really have an upgrade path, whereas Intel will be releasing Ivy Bridge CPUs fairly shortly which you could drop into a socket 1155 board.