I have a Dell XPS 720 and I am getting a lot of stuttering when using Adobe Lightroom. I have a Nvidia GeForce 8600GTS (256MB) in the box now. Wondering if I should add another 8600 SLI or chuck the 8600 and just go a 8800GT (512MB) for the money. Much appreciated!

What is Lightroom? Never used it, sorry. If it's a photo program, I'm thinking your gts should be ok for that. What you might want to look at, how much system memory do you have? May want to consider more ram.

Yep.. Lightroom is just a photomanager with some ability to edit photos.. pretty much like Picasa or Adobe elements.. I have about 3,500 jpeg in the catalog now.. system really running slow as of late. I have 4GB of ram and running a Q6600.

Well I think a single 8800GT would double gaming performance, adding another 8600GTS would add 30 percent or so.

But thats for games. If you can sell the 8600GTS, then definately go with the GT.

First of all however, make sure that Lightroom program even uses the GPU.

These are the system requirements for that program

Intel® Pentium® 4 processor
Microsoft® Windows® XP with Service Pack 2 or Windows Vista™ Home Premium, Business, Ultimate, or Enterprise Edition (certified for 32-bit editions)
768MB of RAM (1GB recommended)
1GB of available hard-disk space
1,024x768 screen resolution
CD-ROM drive

Not very demanding and states nothing about video requirements. I think your problem is with something other than the video card.

Defrag you pagefile,clean out unused Registy entrys,defrag your Registy and then defrag the HD.
Run chkdsk FIRST!