Of course you go with the lens.
You said it yourself - you do most of your shooting with your FA43. That means the kit lenses aren't cutting it.
The FA77 is vastly different than the FA43. It shoots well wide-open at f/1.8 (compared to the FA43 which, as you know, should at least be stopped down to between f/2.2 and f/2.8). The K10D (like my K200D) has beautiful output, so you'll be able to take full advantage of this lens.
After you get at least a couple more good lenses you have permission to upgrade your body
If you're desperate for high ISO performance, a K-x or K-r will do just fine. But you really need a few more lenses. Even if you can't get top end yet, at least get some mid-range models (e.g. DA16-45 or DA17-70) before you go for a K-5 or better. And keep in mind that the color output from the K10D/K200D's 10MP CCD sensor is unmatched by anything since them.
Nevertheless, the FA77, more than any other lens I know of, will transform your photography. The K-5 can never do that.
In fact, a couple of years ago, I was waiting for my new K-5 to arrive just before a shoot on the Universal Studios Backlot. It didn't get there in time, but I'd already sold my K-x, so I used my K200D. The shots I got with my K200D and FA77, K200/2.5, DA*50-135, and other top Pentax glass were noticeably better than what I got with the Canon 5D Mk II and 16-35 and 24-70 L glass I borrowed. The K-5 wouldn't have been an improvement.
(Actually, the A100/2.8 non-macro (also available as an M lens) began the transformation for me. It's almost like a cheaper, slower MF version of the FA77. But the FA77 will allow you to fully turn the corner).