ASICs are very real, but just extremely limited in supply at this early stage. Avalon is the first ASIC to have shipped to customers, and Butterfly Labs is looking to ship soon. 1 GHash/s = 1000MHash/s - so a 60GH ASIC is around 100-120x times faster than a 7970. Yes, they are that fast! ASICs are so fast compared to GPUs due to them being "application-specific" (it's in the name!) - the entire chip is dedicated to computing SHA-256 hashes, and SHA-256 hashes only. It's an efficiency/flexibility tradeoff - compare CPUs and GPUs, for example.
Mining with GPUs (or even CPUs, for that matter) is/would be still worthwhile if electricity was free. After all, hashing power is hashing power, be it small or big - more hashes/s means more chances to win. Of course, electricity is never free, and power efficiency becomes a factor in determining which devices should be kept running, and which should be retired. CPUs have already seen the end of days as far as mining goes - even the extreme multi-core chips - as their high W/hash, combined with the skyrocketing difficulty, simply makes it not worth the while. GPUs still remain viable, but older models are not general purpose enough to be used for mining - the x800 is one of those, sadly.
If you just wish to see how mining works, then it is perfectly fine to give your CPU a whirl - just make sure to mine in a pool (preferably PPS - otherwise you may not see results at all!). As for buying 7970s, I would not do this unless you were planning on using them for gaming too, with mining as a secondary purpose. With the advent of ASICs and increasing difficulty, you may never make back your investment, unless you rely on BTC appreciating in the (very) long-term.