1. A7N8X would unlock your Barton, but I read it somewhere saying that new Barton has different packaging and is factory locked which prevent it from unlocked, even on nForce2 chipset. If yours Barton is unlocked, 200MHz FSB (400MHz DDR) is no sweat. I've seem some people make it to 250MHz (that's 500MHz DDR!).
2. The difference between 9800P and 9700P is not only the clock speed. The hardwired features llike Pixel Shader 2.1 support and infinite pixel rendering will not able to achieve on 9700 as they are two different core R300 and R350. R360 is still in the mist though...
If you want to overclock better, I suggest you get either Epox 8RDA+/8RDA3+ or Abit NF7/NF7-S, there are cheaper than Asus and overclock better. And I would really suggest that you get at least PC3200 RAM because PC2700 RAM will certainly limit your overclockability for sure. Corsiar PC3500 and Kingston HyperX PC3500 are good candidate for overclocker. Hope this helps
I have the a7n rev 1.06 and a 2500 running at 2.25, with a stock fan and voltage. I have heard that the newer 2500s can oc higher but I haven't really pushed mine. Make sure the board is rev 2 as the fsb is more stable above 200.