I would try here...
http://www.pcclub.com/product_browse.cfm?cat=Video%20Cards&subcat1=PCI%2DE%20Video%20CardsThis is the current grand daddy of all PCI-E cards but It will cost you .
http://www.pcclub.com/product_details.cfm?itemno=A9606884Product Description:
Features:
Memory Interface: 384 bit
RAMDACs: Dual 400 MHz
Memory Bandwidth: 86.4 GB/Sec
Fill Rate: 36.8 billion/sec
Graphics Core: 575 MHz
Chipset: GeForce™ 8800 GTX
Clock rate: 575 MHz
Memory Clock: 1.8 GHz
Dual Link DVI - Supporting digital output up to 2560x1600
Chipset: GeForce 8800 GTX
Memory: 768 MB
Bus Type: PCI-E
Memory Type: DDR3
Memory Bus: 384 bit
Highlighted Features :
SLI ready , HDTV ready , TV Out
NVIDIA® unified architecture with GigaThread™ technology:
Massively multi-threaded architecture supports thousands of independent, simultaneous threads, providing extreme processing efficiency in advanced, next generation shader programs.
NVIDIA® Lumenex™ Engine:
Delivers stunning image quality and floating point accuracy at ultra-fast frame rates.
Full Microsoft® DirectX® 10 Support:
World's first DirectX 10 GPU with full Shader Model 4.0 support delivers unparalleled levels of graphics realism and film-quality effects.
Double Lifetime Warranty:
Now you-and your card's next owner-are both covered by the XFX "shield" of protection. Should something go wrong with your Series 6 or greater Graphics Card, and you've registered the card with us online, we'll cover the cost of repair and service-absolutely free!
Dual 400MHz RAMDACs:
Blazing-fast RAMDACs support dual QXGA displays with ultra-high, ergonomic refresh rates--up to 2048x1536@85Hz.
Dual Link DVI:
Capable of supporting digital output for high resolution monitors (up to 2560x1600).
NVIDIA® SLI™ Technology:
Delivers up to 2x the performance of a single GPU configuration for unparalleled gaming experiences by allowing two graphics cards to run in parallel. The must-have feature for performance PCI Express graphics, SLI dramatically scales performance on over 60 top PC games.
PCI Express™ Support:
Designed to run perfectly with the next-generation PCI Express bus architecture. This new bus doubles the bandwidth of AGP 8X delivering over 4 GB/sec. in both upstream and downstream data transfers.
16x Anti-aliasing:
Lightning fast, high-quality anti-aliasing at up to 16x sample rates obliterates jagged edges.
NVIDIA® PureVideo™ Technology:
The combination of high-definition video processors and NVIDIA DVD decoder software delivers unprecedented picture clarity, smooth video, accurate color, and precise image scaling for all video content to turn your PC into a high-end home theater. (Feature requires supported video software.)
OpenGL™ 2.0 Optimizations and Support:
Ensures top-notch compatibility and performance for all OpenGL applications. NVIDIA® nView® Multi-display Advanced technology provides the ultimate in viewing flexibility and control for multiple monitors.
NVIDIA® nView® Multi-Display Technology:
Advanced technology provides the ultimate in viewing flexibility and control for multiple monitors.
Mid Range for NVidia would be this...
http://www.pcclub.com/product_details.cfm?itemno=A9602820 at $254.99
Performance
NVIDIA GeForce 7950 GT
560 MHz GPU
24 Pixel Pipelines
400 MHz RAMDAC
Memory
512 MB, 256 bit DDR3
1450 MHz (effective)
46.4 GB/s Memory Bandwidth
Interface
PCI-E 16X
DVI-I, DVI-I, HDTV-7
SLI Capable
Resolution & Refresh
240 Hz Max Refresh Rate
2048x1536 x 32bit x85Hz Max Analog
2560x1600 Max Digital
Requirements
Minimum of a 400 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 20 Amp Amps.)
Minimum 500 Watt for SLI mode system.
(Minimum recommended power supply with +12 Volt current rating of 28 Amp Amps.)
An available 6 pin PCI-E power connector (hard drive power dongle to PCI-E 6 pin adapter included with card)
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Theres other options between them, but IF your running 256 MB's, the only thing I would look at would be does the card have shader 3.0 abilities. I also just posted NVIDIA cards, since that's what you have right now.
Stephen