preformance ? for fsx

OK im new here so please forgive if I'm posting my question wrong. I purchased FSX Deluxe Edition 2 days ago and am absolutely stunned by the improvements over FS9, but like most other people I'm also getting low framerates 5 to 15 FPS (with all graphics settings at about 3/4 of max). My system specs are Abit AG8 motherboard, Pentium 4 541 3.2ghz HT, 4gb DDR 3200,Nvidia 7800GTX 256mb, SATA 74gb raptor 10,000 rpm(for games) + SATA 150 200gb(for OS), Windows XP Pro, Saitek X45 flight stick, Creative Audigy ZS Gamer. I'm planning on building a SLI system once VISTA and direct X 10 GPU's are available, but that will probably be 6 months down the road. My question is will i improve my FPS more now by swaping the motherboard + processor for a dual core Penitm D or by swapping the PCIx 7800GTX 256 for a 7900gt 512 to get me buy for now. any input on what to swap to improve framerates will be very appreciated. Ive always been a huge FS fan but 5 to 15 FPS makes it hard to fly and 6 months is a long time to wait. TY PS no overclocking on my system anywhere as i dont know how. 
