Hi banana. I'm just about to build a new system, and it won't be that different from yours - Asus mobo, ATi 9800 Radeon Pro...
The CPU I'm getting will probably be the AMD XP2500 or XP2600. I know you can go right up to XP3200, but there's no need. Get one that's on a flat section of the price curve - before it points skywards. Skuzzy has said that the video card will be more crucial than the CPU for Aces High, but you're good on that. My current CPU (Athlon 1.2GHz) works fine in AH most of the time.
The mobo I'm getting will be the much vaunted Asus A7N8X-deluxe. If this is going to be your first Asus board, congratulations - it's a choice I doubt you'll regret. However, before committing to purchase, read the mobo manual for your board. You can download it from the Asus site. The link for your board is here:
http://www.asus.com/pub/ASUS/mb/sock478/P4C800E-DX/e1347b_p4c800-e_deluxe.pdf I would right click that link and then "save target as..." to download it to your PC - better than trying to read the online .PDF - it's 4.46MB. The manual should answer your questions about memory/matched pairs.
As for cases, hehe - I have some experience there. Remember, AMD processors run quite hot compared with Intel. You'll want a well ventilated tower case with the option for 4 cooling fans. I have that and my CPU runs at 50°c. However, that's going to mean more noise... liquid cooling might be possible, might cost a bit. I had planned to put my case in the tower case cabinet of a desk I just bought, and found it wouldn't fit.

Would have been good to press the power button to start up, and then close the door on the noise. If you get a purpose built desk, check the cabinet size against the size of whatever case you get.
Good luck.
