Nill, uprezzing is always done by the printer, no matter what, with inkjet printers [...]
Beware : some drivers use nearest neighbour interpolation, so does my R1800 at least.
So, I see better results sending at least 240dpi to the driver.
I use Lightroom to do so, and it's really transparent to the user : you choose your output sharpening strength, your icc profile, you nail the driver settings once and here you go : you just got to choose the template to get all this set, and hit the "print one" button, perfect results in 2 clicks.
Yes, I miss softproofing though...
But I did try QImage, and (interface set aside ) I didn't felt it was so wonderful : in particular, the default sharpening tasted like rather oversharpened (but it may depend on your sharpening workflow, I use capture sharpening before output sharpening).
And, more closely related to the original question, for big enlargements I prefer a simpler approach (bicubic and some sharpening) to the more convoluted algorithms (genuine fractals et al.) that may meke some parts of the image look better, but give some nasty artifacts or simply bad results to others... Just like I prefer undersharpened to oversharpened.
Matter of taste.