Yeah... The D300, although a nice piece of gear , will not get you the low light performance you need in a dark rink. I shoot a D3 and in the older vintage rinks, I run pretty consistently at ISO 4000 in order to hit 1/500th which is my absolute minimum for hockey. The D300 will present a noise issue there.
My suggestion would be a nice used D700 ....a terrific mid-priced sports oriented camera. Same ISO performance as the D3 and generally can be found for reasonable price used.
When I was using a D200, I never felt that upgrading to a D300 was worth it. Instead, I got a D700 which has a high iso performance that is 2 to 2.5 stops better than the D200. D700 captures at iso 3200 have less noise than D200 photos at iso 800. If you want to stay with DX, you might want to consider a D7000 or D7100. For a used D300 I would not pay more than about $200.
The D300 has about a 1.5 stop advantage over the D200 in high ISO performance. The D7000 has about a 1 stop advantage over the D300. The D700 has about a 1.5 - 2 stop advantage over the D300. For low light sports the D700 would be the best option followed by the D7000, then the D300.
Don't bother with the D300. It's superior to what you have, but a D7000 (or better yet, a D7100) is the real upgrade for DX. As for the D700, none of those recommending that body (which is very good, BTW) have pointed out that you would now need different lenses.
The 85 f1.8 is an OK lens for hockey (I use mine with a D7000, and it is passably long enough, if used near the goal), but it would be useless for hockey on an FX body. Keep that in mind if you use it for other sports, as well.
While I don't dispute that FX is the gold standard for low noise performance, DX makes sports photography work for anybody on a real budget. With a D700, you would need a 70-200 f2.8 ($1,500 used, for the version I) to do what you do now with your 85 f1.8 - and you would lose nearly a stop of light for those REALLY dark gyms/arenas.
On the subject of the 50 f1.8 and 35 f1.8 AFS lenses: if you stay with DX, don't hesitate, just buy them. They're cheap, focus fast enough, and are sharp. Those two lenses, combined with my AFS 85 f1.8, are my indoor basketball kit on a D7000, and my results are acceptable in dark gyms and arenas for 8x10 to 11x14 prints.