what is the save oc for my pd 805

The only way to squeeze it is to keep it cool and make sure you don't pump too many volts into it. I have used several PD 805's (3 in fact) and they all respond poorly to too much voltage. Use the voltage required and no more at any point in your OC. Also, a good MoBo really helps. Of course, it kind of goes without saying, but if you have a good MoBo, you'd be much better off with a low end Core chip then a high clocking PD chip.

A Core2Duo at any speed they come in will be an upgrade from what you have now. A 1.8GHz E4300 scores nearly as well in most benchmarks as a 3.6GHz PentiumD and something the E4300 even scores higher. If you can afford to get an E4500 or E6550 that would be good, but even just an E2180 would be an upgrade, especially if you go the route of overclocking the new CPU.

You can overclock an E2180 and an E4500 to about the same speeds, and the E4500 will have better performance in applications that have heavy L2 Cache use, but they will be similar in performance. The E6550 will have a bit of a lead and an E8000(6MB cache versions) will have a bit more of a lead then that at the same speeds for every processor. Personally, I'd take an E4500 and overclock it, but if you want to keep a tight budget the E2180 would still be a good choice.

Games and encoding use L2 cache, although for games it depends on the game also, some games will scale up with more L2 cache, others will see just moderate improvement. Any of the Pentium Dual-Core E2000s or Core2Duos will see an increase in performance in games, but your video card is also an important factor, what games do you usually play and what video card do you have?

My 805 ran fine for months at 3.6ghz on air and standard volts.
After upgrading though, the CPU was apparently limiting 3d games.

I upgraded the processor and mb, kept the rest, and it made a noticable difference, though the E6850 is only at 3.0ghz.

An E2180 at stock will bottleneck SLI, but shouldn't do too much for a single 9600GT. If you overclock the E2180 to 2.8GHz+ you shouldn't bottleneck at all for 1 card, and it shouldn't even be significant for SLI as many games use more GPU power then CPU. The PD805 will bottleneck a single card and terribly bottleneck an SLI.