Warning: Even the pro's fry very expensive equiptment when overclocking.
Overclocking isnt exactally a stupid idea,
Intels I7 for instance automatically adjust its clock speeds depending on load.
Any GPU you see with "OC" normally after the model number is factory overclocked.
Just not always wise for an end user to do, even with plenty of experiance at it you can fry stuff fairly easy.
Yes your machine would handle the 4870 with ease, but rather than the 4870 check into a Nvidia 9800.
Last I knew they are a bit more money but prices have droped since then also, and they outpreform the 4870 with ease.
I didnt look at the output for the PSU thats in your machine, but dont be surprised if you must replace it if/when you upgrade that GPU.
ATI is also now using CUDA (slightly different name but same basic thing, name offhand is unknown)
And yes BTW, the 9800 is a better card than that GT230, even though the GT230 is newer.
Heat is always a problem, but with a well vented case you shouldnt have a problem with any card on the market.
Didnt look that case over very well,
but I'd assume because it has an I5 that its well withen Intels specs for that & therefore adequate for any available card on the market.
Keep in mind that the 2x slot width of the higher end cards isnt due to the size of the electronics involved.
All that extra space is due to the heatsinks & fan, remove the heatsinks/fan from any & they will fit into 1 slot.
(Thats not saying to remove the heatsinks/fan becuase yes that will fry the card quick)
Have a look at factory water cooled cards and compair that with a air cooled version of the same card for an example.
Was going to add that before but forgot, smaller these electronics get the larger the heatsinks need to be to keep them cool.
Its a catch 22
Are you IT? I've always wondered what your Job Description is.