in terms of quality, Display Port, HDMI and DVI are the same. all three transfer digital signal to be output on monitor. only diff is Display Port and HDMI transfers digital audio signal as well. DVI only limited to video

VGA is analog signal, which means the video quality is adversely affected by length of cable. the longer the lousier unless you have signal repeater. digital cables suffer no such effects

based on the specifications of GT 240, they only have DVI and HDMI (ignore VGA, they should be obsolete by now).  i would suggest going DVI unless you have built in speaker for your monitor which then you can utilise HDMI more

normally i will suggest Display Port over the rest, cause its a non-patented cable (which means cheaper cable, dont need to pay royalty to use the branding like HDMI, which is why HDMI are rather expensive) supports up to 4K reso unlike DVI and HDMI

DVI max reso - 2560x1600 for dual link, single link max 1920x1200
HDMI - 1920x1080, unless HDMI 1.4 which supports 4K but at 30Hz

1. You dont need to have dedicated sound card to use HDMI, you GPU already can encode or pass the audio to your monitor or your home theater.
If your LCD does not have speaker, HDMI still usable, the audio part is optional.
2.HDMI is better on either one.
4. You can plug all the cable to available port on your monitor, it wont explode 

marfccy said

Your DELL monitor does not have build it speaker i think: http://accessories.us.dell.com/sna/product...04&sku=320-2676
btw you dont need to buy any cable, most monitor will give DVI cable for free, some give HDMI (Philips LCD)

Your GPU, GT 240 does not have DisplayPort according to spec. or does it have DP?

Yeah, in fact there is a AMD GPU that feature 6 miniDP, called 5870 eyefinity edition iirc, but not sure if need Active DP adapter or not
Although DP arent meant to replace HDMI but it's better than the rest
http://en.wikipedia.org/wiki/DisplayPort

1. nope, most motherboards/GPU have built in sound card and should be more than sufficient for casual usage. as for if monitor dont have built in speaker, then nothing will happen regardless if you pick DVI or HDMI

2. HDMI is the standard for TV. youll be hard pressed to find a DVI port for most Full HD TV nowadays

3. im using DVI, my monitor came one free

4. nothing happen, youll just pick whichever signal you desire. eg, i have VGA, DVI and DP connected to my U2313HM, all i need to do i open the mini menu of the monitor and select which input source to be used (can be done via Control Panel Screen Resolution as well)
many apologies for confusing you

what i meant was i would normally recommend using DP connector instead of the others. however given that you GPU only supports DVI, VGA and HDMI, i suggested DVI instead

most Nvidia latest gen cards are already including DP ports on them

except the lower ends like GTX650 and below (GT610, 620, 630, 640)