Hi Blackdog,
I have a verry similar system and decided against upgrading. By sheer fortune i got 2GB of RAM now and i dont see any diffrence in AH2. I am not sure what your complain is with lack of detail, if its aircraft detail you can try to set a higher texture size. If you dont like the rough edges try to set AA to 2x or 4x. When you want more performance (get rid of occasional stutter) try to set no AA if you have it enabled or lower it.
To be honest i am not sure if AH2 currently offers more detail that what you already get.
Probably best bet for an upgrade is buy a better cpu, check first which is supported by your motherboard. You might have to flash your motherboard bios with a new one and check the revision of the board. In germany the 939 cpus are already pretty expensive, since a lot of people buy em as last chance upgrade for their comp.
Graphic card is good... dont think youll get much upgrade from a diffrent one without waisting to much money.
You dont mention a sound card. If you dont have one i would get a decent sound card and a nice 7.1 sound system instead of other upgrades. While the performance gain is probably not noticeable the gain in sound quality is quite nice compared to onboard.
Otherwise, maybe i did read your post wrong. Instead of upgrading you could buy a completly new computer, as in buying new motherboard/ram/video/cpu. Then my pick would be getting E6300 with 2GB and ATI X1950pro . As mentioned i really dont know if that gives you that much of an upgrade in ah2. I would rather hope for Christmas then.... better cpus and more choices in video cards, lower ram prices.
For reference, here is the system mentioned by sharkyextreme, note that they have a glitch on their website and call the core2duo system pentium D, probably they copied that from an old article:
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Intel Pentium D System
Case: ThermalTake Armor VA8000BWS Full Tower w/Antec 550W TP3 PSU - $239
Processor: Core 2 Duo E6600 (2.4GHz) Retail - $315
Cooling: Retail HSF - $0
Motherboard: ASUS P5N32-SLI Premium - $215
Memory: 2x1GB Corsair XMS2-6400 (TWIN2X2048-6400C4) - $256
Hard Drive: 2 x Seagate Barracuda 7200.10 320GB SATA - $194
Video Card: GeForce 8800 GTS 640MB - $399
Monitor: Samsung SyncMaster 204B 20" LCD - $345
Sound Card: Creative Labs Sound Blaster X-Fi XtremeGamer - $80
Speakers: Logitech THX Z-5300e 5.1 Speakers - $134
DVD-/+RW: Samsung 18x DVD+RW/-RW SH-S182D - $29
Communications: Onboard LAN - $0
Mouse: Logitech MX Revolution Laser Cordless Mouse - $75
Keyboard: Logitech G15 Gaming Keyboard - $68
Operating System: Windows XP Professional SP2 with Vista Upgrade Coupon - $139
Floppy: Generic - $5
Total: $2,493
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My take on this is that it is far to expensive. Using E6300, X1950pro, diffrent motherboard, cheaper case, smaller hard disks, simple mouse/keyboard i come off 700$ cheaper with still pretty good performance in ah2 and nothing reduced but the whizz factor. The 8800 is the only card that can in theory use directx10 and has by far best performance, but 400$ is a lot for a graphic card. If you need it for new games, ok. But 700$ is already a week of nice vacation somewhere with hot chicks on the beach... uhm i mean good food and wellness offers..