so if PNG is a lossless compression....

then would I get a sharp image if I were to stretch a 800x600 picture and print it on the whole A4 paper?

The two are unrelated.  How sharp an image you'd get depends on your printer and printing software, regardless of image format.

On top of that, you definitely won't get a sharp image if you print an 800×600 picture on an A4 sheet. You'll need something like 2048×1536 for it to look sharp.

You should read the following and follow interesting links:
http://en.wikipedia.org/wiki/Image_resolution
http://en.wikipedia.org/wiki/Image_file_formats
http://en.wikipedia.org/wiki/Digital_image_editing

TL;DR: nope, unless you are quality-blind. PNG being lossless is completely irrelevant. 800*600 is the amount of detail in pixels there is on your picture, and paper squeezes in more detail in less space than a computer screen.

You can try it, but it'll either look fuzzy (bicubic filtering from the resample) or blocky. What you're thinking of is a vector format like EPS or PDF.

Note that PDF isn't always scalable, it can include both scalable and unscalable content.

So... for an average user liek me, I might as well convert all the .PNG HCG files into .JPEG to conserve remaining HDD space?

And what's the point of putting up large sized picture files (as seen on various HCG batch torrents) in .PNG format in the first place anyway?

Because they (PNGs) don't have compression artifacts like JPEGs do. Compressing anything in JPEG, particularly cartoon images or other content with high contrast, fine lines, etc., produces blockiness, fringing and other effects (visibility and severity depending on the level of compression used).

>>7
If you can't afford adding HD space, maybe, but use 90-95% quality. (even with 95% quality it would at least divide by 2 the filesize). If you don't know what you are doing or are not sure how to use batch conversion tools, you would be much better off buying a second HD.

>>7
Try recompressing your .PNGs with pngcrush or a similar utility, if you're that concerned about your HDD space.

If you convert to .JPG, you'll gain space but at the cost of noticeable image quality degradation.  How much your images degrade depends on how much "compression" is used.  Higher compression (or lower quality) will give smaller files, but that smaller size comes at the expense of quality loss and degradation in JPEG, and this degradation can *never* be recovered except by finding or recreating the source lossless image.  If this is what you want then knock yourself out, but make sure you understand the consequences, and make sure you're willing to live with it, before you delete the first PNG.

Alternatively, instead of converting them...  why not just back them up onto a CD-R or CD-RW, and remove from your hard disk the ones you don't want on-hand at all times?  That'll save HDD space *and* let you keep your images in lossless format.