Can an optiplex gx270 run wow?

Depends entirley on the configuration. You have just named a brand, which can be supplied in many different ways.
For example the default (minimum) configuration is a 2.66GHz Pentium D with 128MB RAM and intergrated video. That wont run WoW.

The main things you should look for is at least 1GB RAM and a Video Card that can support pixelshader 2.0. If you can perhaps link some you would pay for...

Yes. The PC I was using before the one I just finished building was an Optiplex GX270 with 2g ram and an agp ATI 9550. Ran WoW with no problems. I turned a couple things off graphically, but before that there was very little lag. I just turned off stuff I didn't care for like ground clutter.

The Video Card is extra so it depends if the original purchaser upgraded the PC.
Be careful giving such definitive answers, if the OP went and bought any old non-upgraded GX270 then he would need to go and spend more money on a compatible Video Card.

EDIT: btw i cant view that link from work, apparantly shopping on eBay isnt in my job spec  

You are right PR2. I hadn't taken into account I already had that video card when I got that PC. I apologize for that part of my answer. With relatively low money, that 270 is capable of running WoW with little problems.

You could run wow on a dented toaster. You would be running it at the minimum settings, but you would still have your foot in the door.

maybe single player questing but forget going into a major city.

Yea would lag like crazy. When i first started playing there was a chap who had a really terrible rig...couldnt move through major cities. He had to set follow on me and i had to run him round the Auction House and Bank and such >.<

if you're looking at used dell systems, you should look at newer optiplex systems like the 320, gx620, 740, 745. Those have PCI express so you can add a video card as long as it doesn't require 450w or greater psu, you should be fine.

gx270's are older and most upgrades to them are more expensive. they take ddr ram, which is pretty much 2x's as much as most ddr2, and they have either pci or agp which finding a good video card can be rough.