2- 9600GT in SLI or 1-GT260

A single 260 and the 'SLi later' option has to be considered as W7 and DX11 card(s) are almost upon us, the 9600's are just not worthy if you have to get two at the same time.

You don't mention what CPU you are running so I'm assuming that it would be able to handle a single 260 at least, which is still the way I would go even if you don't wish to move to W7 simply because it's a bit newer than the 9800 if nothing else and if you are still on XP the option to run Vista with some real horsepower in the graphic department is always open.

Running Athlon 64 X2 4600 (2.4GHz). Current PS is 550 watts. MB is ASUS M2N-SLI Deluxe 2GB ram (2-1GB 800)

19" monitor running at 1440x900

L4D lately and mainly stick to FPS titles (FarCry, BF series, HL2, etc...)

I'd chance an arm on the 260 and just turn up all the eye candy, up to eleven and break the knobs off. Sli on that board is pointless (now, with current cards), I had serious CPU bottleneck issues when I put a pair of 8800GT's on a 3800 x2 @ 2.6ghz.

Not sure of the performance difference between two 9600GT's in SLI compared to a 9800GTX. Cost is roughly the same (2-9600GT vs 9800GTX) so a 9800GTX would not be worth it compared to a GT260.

2-9600GT's would be $139
1-GT260 is $159
19800GTX is $120

I'm looking for the best cost/performance and the 9600GT is top of the list of the Best July 09 video card list (under $100)

I suppose I could also get a single 9800GT for $89 and still maintain decent performance.

I'd steer clear of the recertified card, it only has a one year warranty.
I would not advise using the twin 9600 setup. Even now, not every game will benefit from SLI, although most do, but I'd still prefer a single card over two low/medium end cards, it's more consistant and easier to manage.
Which of the others is a matter of personal choice: The 9800GT is the closest match to the CPU and still a fine card but it'll lack the horsepower to use max settings with AA in more demanding titles.
The 9800GTX would be my choice: It's a little cheaper than the GTX260 and has enough power to run AA/AF and high graphic settings in demanding games.
The GTX260 has the advantage of being able to apply high AA/AF AND run PhysX without suffering too much framerate loss: All those features are done by the card itself and will not be effected by the 'fairly' slow CPU. If yopu want PhysX and all the bells and whistles and enough power to have silken framerates in even demanding titles (CPU limitations asaide) then the GTX260 is the card for you.
BTW, which motherboard do you have? Have you considered a CPU upgrade?

^ Given your requirements, that seems like a good choice.
As a matter of interest, the motherboard supports quite a few of the newer AM2+ processors, so you may wish to consider a CPU upgrade in the future, rather than a whole new setup:

http://www.asus.com/Product.aspx?P_ID=szSFtrap7crpBaQE

Bought the GTS250. I just don't game that much anymore so a new system is not cost effective. Trying to stretch every little bit out of this system without spending much money. Thanks for the input all.