You should remove the heatsinks, processor, memory, add on cards, cabling, insulation, everything from the board. I've never seen anyone stick the board in the oven in cold - every guide and thread I read they've all preheated to 385F.
If you want to go through buying a heatgun, you can probably try the IBM guide on your GPU too (considering that most of the time, the GPU is at fault). 230C seems a bit hot to me, but I've seen people recommend 450F for 4-5 minutes when doing oven reflows. I've tried it once (without success) after a 385F reflow on my GPU I mentioned before. It didn't melt any of the components, again, just bloated some capacitors.
Weigh in buying a heatgun. If you think it's worth it, get one, do your GPU only, see if it works, and if not, you can always resort to sticking her in the oven. The beautiful thing about broken parts is just that. They're broken. No harm done if in attempting to fix it you fail.
About cooling - most people take the board right out of the oven and set it on the counter to cool, but I wouldn't. Like the IBM guide says, solder is pretty fragile when liquefied, and moving it about isn't a good idea. I just open the oven door a bit and let it sit for a while.