I'm planing of taking the following components:
MB: ASUS P5Q
2x (RAM PC-6400, 4 GB, CORSAIR XMS2-6400, DDR2 800 MHz, KIT 2x2 GB) = 8Gb of RAM
and a quad core processor.
Now, I'm not sure which which processor should I take, Q6600 or Q9450. And I'm not sure because I wanted to overclock the processors to 3-3.2Ghz.
I'm seeing that lot of people have a problem in understanding the FSB and the multipliers and when I think that I got it, something else pops up.
Anyway, as I understood, the motherboard Asus P5Q supports 1333Mhz bus. Q9450 has a 8x multiplier, and to get to 3.2GHhz, you need to set bus to 400Mhz, 400x8=3200MHz.
For a Q6600 has 9x multiplier and to get 3.2Ghz you need bus set to 350MHz.
As for RAM it should work in both cases, because in worst case (with Q9450) it is 2(dual rate)x400=800Mhz.

And now I've heard that there could be problem with RAM if I put 8Gb and that part I don't understand. Should these configurations work? If so, I would probably get Q9450.

Both will work just fine. It's harder to overclock 8gb of ram than 4 or 2gb, but the 800mhz ddr2 don't need to be oced with such oc. Although with q6600, you should ideally run it at 9x400=3.6ghz at 1:1 ratio for the 800mhz ddr2 ram. It's the most common q6600 setup, simple 1 minute oc with everything balanced and rounded off.

Could you explain to me why is it harder to overclock 8Gb of RAM than 4Gb?
Also, are you suggesting that I should take q6600?
I would like to overclock the system but my needs are quite brutal, 24h/7d/365d all four cores on 100% running on Linux server. I don't want it to be unstable or having overheating problems. So I wouldn't go for to much overclocking. As understand q9450 doesn't heat that much when overclocked and it should have some better performance.

I would get the Q9450 if you have the cash, as it runs cooler, has more cache.

The simplest overclock for Q6600 is 8X400, that way your DDR2 800 memory is running right on spec at a 1:1 ratio, depending on the one you get, you could do this overclock with all the voltages at default. If you want to clock to 3.6 vice 3.2, then a 9X400 works fine, however you will probably have to make some adjustments to voltages.

the q9450 is the better processor and can oc like the q6600 and it runs a little cooler so get that if you can. 8gigs of ram are harder to oc because all the heat that are going to be coming off the sticks. just get expensive ram with good heatsinks and you can always try this http://www.newegg.com/Product/Prod [...] 6835704001

OK, thank you all. 2. more questions:
1. do I need a new cooler, or is the stock one fine for 3.2/3.6Ghz?
2. is there some benchmark q6600 vs q9450?

The reason 8gb of ram is harder to oc doesn't have as much to do with heat as just the load on chipset. A highly capable chipset like x38/48 will have little trouble, while a bad chipset like 680i may not be able to run some models of ram in 8gb at all.

1. Yes, you need an aftermarket cooler for 3.2ghz, q6600 or q9450. Especially since you're running it 24/7.
Top 5 performing coolers:
http://www.frostytech.com/top5heatsinks.cfm

2. Q9450 average 10% faster than q6600 per ghz. Even though it's just a die shrink and architecture is identical, it use new instruction set and larger 2x6mb instead of 2x4mb cache, which makes up the speed increase. This, of course, varies depending on application. For encoding, it would be more than 10% per ghz, for gaming, less.

OK, thank you very much!