i need a new computer, and wonder what is the best buy as far as RAM and processor speed goes to run gnu optimally

CPU Architecture
For rollouts, or even 4-ply evals in GnuBg, you need all the processor speed you can get. That means Sandy Bridge or, if you can afford it, the newer Ivy Bridge architecure from Intel.

CPU Speed
You need as fast a clock as you can afford. CPU speed is measured in GHz. The bigger the number, the better. Don’t scrimp. I am not a fan of overclocking. So, buy a fast clock, and then you don’t need to overclock.

Overclocking
Did you know that the different clock ratings for CPU chips that are otherwise identical are not due to different manufacturing techniques? There is only one fabrication technique. After manufacture, when the chips are tested, Intel cranks up the speed until they fail. The ones that run faster before failing are then rated for faster clocks, and are sold for $200 to $400 more.

You can see why I am not a fan of overclocking. If Intel tests a chip, and determines its maximum clock rate, why would you want to run it faster? If it could be run faster, Intel would have rated it higher, and sold it for several hundred dollars more.

We are not talking about certain failure every time you run your overclocked chip. What happens is little screw-ups occur once in a blue moon. Perhaps a program accesses a CPU register too quickly, before the internal voltages have settled down to stable levels. Or your computer freezes slightly more often than the usual PC or Mac. Most users of overclocked chips will simply reset their computer, and never make the connection between overclocking and the occassional computer freeze.

Multiple Cores and Threads
For GnuBg, multithreading capability is limited. If you learn how to do batch rollouts, however, GnuBg can take advantage of mulitple cores and threads. If you only do one rollout at a time, GnuBg will not take advantage of multithreading. That means you may not need to buy a hexacore machine!

If you plan on shelling out an extra $75 to buy XG, then you want all the cores and threads you can get. XG will use up to 64 threads, even when only one rollout is underway.

RAM
Memory is much less critical. Backgammon bots do not require large amounts of memory. I believe you could get by with as little as 2MB if all you wanted to run were GnuBg. That said, I do not recommend buying any computer that has less than 4MB of the fastest RAM that the motherboard accepts.

Hard Drive, Monitor, etc.
Backgammon bots do not require anything special in these departments.

I hope this helps. Perhaps those with more experience than I will fill in the specifics. I buy a computer once every couple of years. Those who work in IT often purchase dozens or hundreds every year. They can stay on top of developing trends much better than I can.

Good luck.

This is not entirely accurate. Gnu can use only one core in interactive play (and its "hint" feature) but multiple cores will be used for rollouts or match analyses.

Thanks, PLM.

Do you know of anything I can read to learn more about this? Rollouts still seem to take a ton of time compared to XG.

For GnuBG (or XG) 99.9% of the speed is coming from the CPU. Mike did a great job at breaking down the different part.

What I always recommend is to check http://www.cpubenchmark.net/high_end_cpus.html

For XG (and also for GnuBG) the speed will be linear with the passmark CPU score. So a core i7 3770 (4 core ivy Bridge) will be 1.05 faster than a core i7 970 (6 core sandy bridge) (10401/9874).

Disclaimer : the following is advertising (although accurate) :
XG is indeed much faster than GnuBG (about 4 times faster for Rollout) on a up to date system (quad core). check http://www.extremegammon.com/Compare.aspx .

Also Rollout converge much faster as the extra accuracy as the superior Neural network helps get the variance reduction to be more accurate.

And while i am in a advertising mode, do not forget to check XG mobile for the iPhone and iPad at www.xg-mobile.com

I don't know any good summary about this, the online documentation at www.gnubg.org is out of date.

The number of threads to use is set in Settings|Options|Other|Eval Threads. You should not set it higher than the number of cores in your machine.

Besides that, my experience (under linux) is that on an old dual core system a gnubg process running with 2 threads was very noticeable from other interactive usage (but fine for overnight rollouts), while on a Core i5, a rollout or 4ply analysis running with 4 threads doesn't harm basic GUI/editing/etc... usage. You'll have to experiment a little and see what suits you.

There must be some discussions about multiple threads issues at http://lists.gnu.org/archive/html/bug-gnubg/.

Regarding the speed gap compared to XG, as Xavier pointed out, the latter is substantially faster anyway.