It seems there is no problem whatsoever with P4 3.0ghz and any current cards out there now. To answer about R520 and G70, you'll have to wait 8 days and until july 26th for ATI. 3.4ghz wont make any difference for you for any current generation cards. Besides if I was you, I'd just try to overclock there.
You see even with current cards such intense games like Doom 3, Far Cry, FEAR (coming out) and so on are still graphics card limited at 1600x1200 4AA/16AF. So cpu speed doesnt matter at that point since the graphics card is the main driver there. Again you'll have to wait a bit to see how much performance you'll get from G70/R520. I am guessing about 80% of the performance increase. 20% will be a likely penalty against say having FX57 processor. Of course this figure will be like 50% at 640x480 given A64 prowess as a gaming cpu. Of course who plays games at such settings???? Again, it'll be up to you to decide whether it's worth it to spend $300+ on a new graphics card for 80% performance out of 100% of its potential (sorry if i am confusing you). In other words if the card is only 40% faster than x800Pro in optimal settings (ie paired with the fastest cpu out there), I probably wouldnt' upgrade since your system is fast enough. If the cards are 100% faster, then say if u essentially even get 80% out of those 100% (due to slightly slower cpu), I'd say that it's still worth it.
Some games are more CPU limited than others. I played back a Half-Life 2 timedemo (that I recorded in a scene with heavy AI and lots of geometry) at low video detail, and then again at medium detail and the average was about the same or 1-2fps differance. But if you're going to be using high-res with AA/AF then a 3GHz P4 is perfectly fine. Your CPU could last another year or more easy.
definately a waste of time to upgrade from 3.0 to 3.4 MHz. Your cpu is not holding you back. I can imagine it might bottleneck the fastest next generation cards, but a 3.4 Ghz cpu will as well. Bets bet is to overclock your current cpu a little bit - should have no problem at least getting to 3.2 GHz.