Well, first of all, let's clarify your CPU model. I remember you posting in the P4 owners thread and mentioning a Pentium D 820. The motherboard you specified now, however, is PGA478-based, so it can't be a Pentium D. It's either the P4 northwood or the prescott. I'm assuming it runs on FSB800 and includes HT as well; am I right?
Anyway, you should be able to get some overclocking done on that board, but don't expect too much - PGA478 and, at least, the early LGA775 ASRock mobos have no voltage control and lack other handy overclocking features as well. Depending on your chip, you might be able to reach ~3.2-3.4GHz stable tops. I wouldn't expect any more than that.
To overclock the 9800 PRO, use ATItool or ATItraytools. I'm not 100% sure about their compatibility with windows 7, but it won't hurt to try.
In regard to RAM, the sweet spot for PGA478 rigs these days is 2GB of DDR400.
If you're considering getting a kickass AGP graphics card as well, you would want to look at the ATI HD3xxx-4xxx series. The 4670 & the 3870 are the best cards, but your CPU will greatly bottleneck them. A cheaper solution would be a 4650, or even a 3650.
The best solution would be to save up for a new rig, though. Once you upgrade your 478-based rig to the limit and realize that you can't improve anything else, yet the performance is still severely lacking, you finally notice the futility of it all. I've been there, mate
Just about half a year ago
A budget solution would be the ASRock DualVsta motherboard, which supports both DDR & DDR2 and AGP & PCI-e (not simultaneously, though), which would allow for gradual upgrading.Edited by Hyoketsu - 12/21/11 at 3:32am