and if you did have dual 9500GT's running in SLI you can only use 1 display, unless thats changed?
you'd have to run them as separate cards to achieve 3-4 display setup.
Driver wise, they all use the same drivers, Geforce 8xxx, 9xxx, etc, series. so thats not an issue.
to want to run a 3rd card down the road, well... if your looking at 3 PCIE cards, you'll need 3 PCIE x16 physical slots to achieve that. all cards run independent of each other.
as rich said, was going to point that out too but then scrolled down and saw rich posted it too, we need to know the specs of the motherboard in the system first, 1 or 2 PCI-Express slots (which types), PCI slots etc?
as to the difference between the two 9500GT's you linked to, the Asus is a better deal from a Performance standpoint, $5 cheaper, 100MHz faster core clock, Mail in rebate so its even cheaper and Free shipping. but as you pointed out its a dual slot card thanks to its cooler (pathetic thing is, Asus probably doesn't need the dual slot cooler, as this 9500GT is running close to the same specs as a previous 8600GTS it replaces and many of those were single slot cards)
if space is an issue I'd go for the XFX (also in general XFX has the better warranty, along with eVGA & BFG).
but until we/you know the specs of the motherboard and slots you have as well as spacing between slots etc, would hold off on jumping on anything just yet.
as to running 4 monitors, I honestly don't see why 2 9500GT's would have a problem running those resolutions on 4 displays (2 per card), as long as your not gaming on those resolutions on more than 1 display, for Photoshop and other more "mundane" tasks on the PC, 2 9500GT's shouldn't have much trouble running 4 displays. why you'd need to add in a 3rd similar spec card is beyond me. maybe its just me, but it seems a bit overkill and unnecessary for a quad display setup.
since your looking at running more than 2 cards, SLI is out due to lack of ability to run multiple displays in SLI mode (and no specs on whether the mobo can even run SLI), also since your new PSU can more than handle it. maybe getting a single beefier card for the main display device, and a 9500GT for the secondary.
a Geforce 9600GT or 9800GT plus a 9500GT for secondary would make sense to me, the primary being plenty to run games on, and should be enough to run Diablo 3 pretty good when it comes out, as well as handle the resolutions your talking about fine too.
your PSU has 2x PCIE connectors, a 6 pin and 2x 6+2 pin, enough for single connector cards. 9500GT shouldn't need one. and that would mean there's plenty of connectors for something as large as a 9800GTX+/GTS 250 there. it has 3x +12V rails, each @ 25Amps, but there's no way that PSU will be able to max out all those amps on that 525W rating, 75Ax12V= 900W
way past its power specs.
Newegg.com - ENERMAX PRO82+ EPR525AWT 525W ATX12V Ver.2.3 SLI Ready CrossFire Ready 80 PLUS BRONZE Certified Active PFC Power Supply - Power Supplies
with a $160 budget, something like this is possible (if enough room in the case)
Newegg.com - ZOTAC ZT-98GES5P-FCP GeForce 9800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Desktop Graphics / Video Cards
(just get a cheap HDMI to DVI converter for the HDMI connection and you have 2x DVI's)
Newegg.com - MSI N95GT-MD512-OC GeForce 9500 GT 512MB 128-bit GDDR2 PCI Express 2.0 x16 HDCP Ready Video Card - Desktop Graphics / Video Cards
comes to total of 2 cents under $160, not including shipping, factor in $30 of Mail In Rebates...
run the 23" display on the 9800GT for your games and any future games, and run the 2 19" displays off of the 9500GT, both cards should be using the same drivers set, so shouldn't be an issue of driver versions conflicting.
but this is of course assuming you have 2 PCI-Express x16 Physical slots in your system. if its just 1 PCIE x16 slot and PCI or PCIE x1 slots, then that changes things a bit. PCI 9500GT's are expensive compared to PCIE versions, and x1 PCIE cards are few and far between, low low end, and EXPENSIVE.