Following is the fsx.CFG I used for my GigaByte Nvidia GTX285.  Nvidia driver used is 8.17.12.5896 with nHancer 64.2.5.9
Average framerate achieve is 39 on my Dell 30-inch.

[JOBSCHEDULER]
AffinityMask=14     // 14 for last 3 cores — No need setting to 15 for 4 cores
[BufferPools]
PoolSize=1000
RejectThreshold=126976    //124Kb = 126976
[GRAPHICS]
ALLOW_SHADER_30=1 // === Important to assist PoolSize ===
HIGHMEMFIX=1 // Fix errors of black squares around lights whilst panning around
STALE_BUFFER_THRESHOLD=1024 // 1024 in KB
[Display]
TEXTURE_BANDWIDTH_MULT=80
UPPER_FRAMERATE_LIMIT=47
WideViewAspect=True
TextureMaxLoad=12 //Use multiples of 3 ONLY but may induce stutters
[DISPLAY.Device.NVIDIA GeForce GTX 285.0]
Mode=2560x1600x16
Anisotropic=1
AntiAlias=1
[TextInfo.1]
Latitude=1,1
Longitude=1,2
Altitude=1,3
Heading=1,4
AirSpeed=1,5
WindDirectionAndSpeed=1,6
FuelPercentage=1,7
LockedFrameRate=2,1
FrameRate=2,2
AverageFrameRate=2,3
[SlewTextInfo.1]
Latitude=1,1
Longitude=1,2
Altitude=1,3
Heading=1,4
AirSpeed=1,5
LockedFrameRate=2,1
FrameRate=2,2
AverageFrameRate=2,3
[DynamicHeadMovement]
LonAccelOnHeadLon=-0
LonAccelOnHeadPitch=-0
RollAccelOnHeadLat=0
YawAccelOnHeadLat=-0
RollAccelOnHeadRoll=0
MaxHeadAngle=0
MaxHeadOffset=0
HeadMoveTimeConstant=0
[SCENERY]
LENSFLARE=0
DAWN_DUSK_SMOOTHING=1
IMAGE_COMPLEXITY=5
[TrafficManager]
AirlineDensity=10
GADensity=30
FreewayDensity=20
ShipsAndFerriesDensity=0
LeisureBoatsDensity=0
IFROnly=0
AIRPORT_SCENERY_DENSITY=5
[TERRAIN]  // ===================================<–<<–<<
LOD_RADIUS=4.500000
MESH_COMPLEXITY=100
MESH_RESOLUTION=23
TEXTURE_RESOLUTION=29
AUTOGEN_DENSITY=5
DETAIL_TEXTURE=1
WATER_EFFECTS=4
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=250 // 4500;  450=Optimized
TERRAIN_MAX_AUTOGEN_BUILDINGS_PER_CELL=1000 // 3000;   1000=Optimized
SWAP_WAIT_TIMEOUT=10 // 1 – 60   Remove texture in solid color but may induce stutter and slow down framerate