Go dual core. The video card makes much more difference in FPS than the CPU. LIke was said, the difference is only minutely measureable, and won't be noticed. Should you decide to work with some video from that vaction, or some specatator even you filmed, you will appreciate that dual core very much.
If you want to really OC it, go with water. It's worth it. I would also opt for 2GB of high quality ram on a high quality motherboard. If you've built systems before and overclocked, you know how important a good motherboard is. You can put the best hardware money can buy on a board, but if the board is crappy, you'll never see the full potential of it.
You'll also want to mkae sure you get a good quality 500+ watt PSU with strong rails of at least 30amps or more on the 12+ rail. OCZ and PC Power & Cooling are good ones.
If you want to go dual core, go all the way. You won't see the full potential unless you match all the components, and use good high quality hardware on everything.
If money becomes an issue, it would be better to go with a 6xx CPU and get the better hardware, then it would to get the dual core and skimp on something.
There are also new CPU's coming out 1st and 2nd quarter of 06 that are really amazing. If it's something you can wait on, you may want to look at those options as well.
There's lots of great things happening in the Intel world right now, and it's going to get better and better.
Edited to add: Take a Lokk at the Asus boards. For the 775 series CPU there is not a better board out there right now.
Also, bump the RAM up to 2GB. There are good 2GB kits that OC very well now. I have a Corsair XMS 1GB kit and it will only barely do 230FSB. It has Infineon Chips on it, and I hate it. It's the worst OC'ing Corsair I have ever owned. If you go with Corsair, make sure you get them to tell you what IC's are on it *before* you buy it. They played a dirty trick with the 1GB kits. They originally had BH5 chips on them, and then on only two versions of that same ram threw in some Infineon chips, and that stuff is horrid! I hate to say that, but it's the truth. I thought it was a pretty dirty trick, becasue they were selling this stuff based on the rep of BH5 which is why I bought it, an didn't tell anyone of the change until after I questioned them about it. It was not on their forum at all. They had a thread that showed all the chips on the different ram available except the ones they stuck that Infinieon on.
Also check out nVidia's 6800GT's or 7800GTX's. Those things rock!
I don't kinow how much you want to spend total, but the above stuff is all first class stuff. A system like the one above that I suggested will really kick some major butt. I have a freind on here that has basically that same system with a 6xx CPU and that machine is awesome.