I would absolutely wait to purchase a new card until the R520 is released, and any surprise from nVidia, as well. Hopefully, this will take place later this Summer/Early Fall. By the way, you don't have to wait until ATI releases their high-end card in early 2006, because I can already guarantee to you that it will be a 32 piped card with clock speeds surpassing any other card.
Honestly, if money is an issue, the 7800 GT seems attractive at this point (even though it hasn't yet been released). Compare it to whatever ATI releases as a competition, and whichever performs best, get it.
For next generation, I wouldn't get anything lower than a 24 piped card, because heck, you could just get a 6800 Ultra right now. Although the low-end R520 is supposed to feature SM3, so perhaps rethink your options on that.
The only advise I can really tell you at this point would be to just wait things out...I think you can manage just a couple more months with your 9800 Pro. Once ATI releases their cards, making a decision will be *much* easier. Right now, though, there's not much concrete info to offer.