Overclockers.com has an article on your pc, which I found doing a yahoo search. They use the conductive paint trick, connecting 2 pads for overclocking. I recommend you post your question in the dell forum; someone must have upgraded the cpu by now. I would recommend the e7200 or e8400 based on your bios date. Anything less would be a waste of money.
If you are inexperienced in changing components, and need your dell warranty, I recommend you leave your system alone. And whatever you do, don't tell any dell representative you've changed anything in your system or dell may void your warranty. If you decide to do a decent cpu upgrade, you may have to remove a shroud over the cpu heatsink. Dell uses a passive design on some systems, with the cpu fan on the back of the case. You'll need to use a retail box Intel heatsink to replace the dell heatsink, and learn about the Intel push pin design of the cpu heatsink. It requires some force to install properly. Good luck.
I am a Dell Optiplex 745 owner that was in the same situation.
I started with a Pentium D 3.4 then upgraded to a E4300 1.8
Using what is called a BSEL mod which another poster mentioned about
connecting to pins on cpu with conductive ink I OCd the
E4300 from 1.8 800fsb to 2.4 1066fsb
Well I wanted more speed so I just purchased a E4500 which is 2.2
that will OC to 2.93 (basically 3.0 ghz) and it cost me about $50 USD
Now I am not sure how much 50. GBP is compared to USD
but if it is close than the E5700 wouild be a great upgrade
The problem is that though you can support a pretty fast CPU from the list
in prior post like a Q9550 (if that is true) then that is about the most powerful
CPU yu can get BUT the cost factor in buying a C2Q Q9550 right now isnt worth
They go for over $200 USD easily SO getting a cheap E5700 at 3.0 is
best performance per dollar deal as MrBullet wisely suggested.
Later on when the E5700 which would be a MAJOR improvement over your old cpu E2140 1.6.
You will have basically double the performance you have now cheaply
THEN later on in a year or two when hopefully the higherend C2D/C2Q drop
in price if the CPU market follows the standard pricing pattern in the past
well then you could upgrade to a high end Core2Quad which would blow away the E5700 and again be good for awhile performance wise
And remember you can always sell of your old CPU on Ebay or Craigslist etc
and get some money back on the upgrade.