Often players ask what new PC is sufficient for the Aion, do we need QuadCore cpu and so on, I hope these questions will be answered for you here.
1. Will Aion use my new QuadCore cpu?
Most likely not, since FarCry engine which is used in Aion doesnt really use more than two cores:
1 vs 2 vs 4 Core is 100% vs 118,9% vs 120,9% at 800x600, and the difference will be lower the higher resolution you will play on.
Intel Core 2 Extreme QX6700 (Quad Core) - BeHardware
2. How much memory will I need?
2GB is sufficient, as FarCry uses about 500MB of memory itself, just add OS and background programs. In Vista total memory used with FarCry is 1,2GB, less in XP.
NEW 3. Whats better - faster or more memory? Like 2x2gb ddr2@800mhz or 2x1gb ddr2@1066mhz.
At lower resolutions (1024x768) faster memory gives about 5% more fps, at higher res difference almost dissapears. For more details:
A-data 2GB DDR2-800 Extreme Edition memory
The best bang for the money and more future proof is 4GB of 800MHz, but if you can afford it - 4GB of 1066MHz (or even 2GB of 1066 now and add more later) will be
very slightly better

Its best just to spend price difference on faster video card/cpu, or even better display than faster memory, IMO.
NEW 4. Will Aion run better on my SLI/CrossFire?
Yes, in most cases it will. Keep in mind its mostly drivers related, and it may take some time for AMD/nVidia to catch up. From price conscious perspective its not worth it getting SLI/CF as any faster single gpu card (like 9600GT, sweet price/performance card) will be more than sufficient for Aion. Unless you play on ultra high resolutions with anti-aliasing, want smooth performance even under heaviest loads, then SLI/CF might be the thing for you - scaling for the FarCry engine is very good.
Perfect parts for various budgets:
~250$
AMD X2 4000+, 2GB DDR2, ATI 3850 256MB (check if in your region 9600GT is similar priced).
Perfectly playable:
1) Up to 1680x1050 with AA 4x/ AF x16, 60+fps
2) Up to 1920x1200 with AA 0x/ AF x16, ~60fps
~300$
AMD X2 4000+, 2GB DDR2, ATI 3870 512MB or 9600GT 512MB
Perfectly playable:
Up to 1920x1200 with AA 4x/ AF x16, 60+fps
NEW ~500$ - sweet high-end performance for affordable price
Intel E8400 3.0GHz, 4GB DDR2, Geforce 8800GTS 512MB
For resolutions 1680x1050 and higher, perfect for 22-24" LCD.
Perfectly playable:
At 1920x1200 with AA 4x/ AF x16, ~100 fps
Anything faster than those parts will help you in massive pvp/pve scenes, otherwise will be overkill. Why I mentioned AMD cpu and video cards? They are cheaper than Intel cpu and nVidia cards, which definately have faster products, just costs more. If you can afford them (like another step up - E8400 3.0GHz, 8800GTS 512) - go for it, just keep in mind
the sum of all parts counts, if you buy super fast 9800 GX2 but have an old/small monitor, whats the use? Its better to buy with saved money nice 22-24" LCD
NEW Tip: if you are planning getting computer for Aion and not in the hurry to buy now, wait till Aion is about to be released - you will get higher performance parts for the same price. For example, in 2-3 months next gen ATI cards will be released, which should offer up to 50% more performance for the similar price. nVidia should follow AMD and release next gen (or at least faster G92 refresh) cards too in summer - autumn.
Other:
* Fast hard drive (7200rpm) and some free space (20+GB).
* Large and quality monitor - whats the use of amazing Aion graphics if we cant see them?

* Any quality motherboard (and other parts), nothing worse than crashing PC during the game.
* Good keyboard and mouse, we spend so much time with them that its really makes the difference.
* Comfortable chair (yes!

)
Further upgrades, if you can afford it:
1. Faster video card and cpu. The higher quality settings during smooth mass pvp/pve scenes, the better.
2. Aion has an upgraded FarCry engine, and since memory is dirty cheap now, you cant go wrong with 4GB of memory either

Just dont forget you'll need 64bit OS to utilize all memory, as 32bit OS will allow you to use about 3GB.
* Disclaimer: Aion has updated FarCry engine, which may slightly increase requirements for the smooth gameplay. With beta testers contributing their results its clear NCsoft will need to polish and tune the game engine, but no doubt they'll try to scale it as much as possible so Aion will be accessible to as many players as possible.