I think it depends how fussy you are about image quality, and how much SD vs. HD you will be using the PJ for.
For me personally, 90% of our viewing is 720p or higher, so SD stuff does not look great at 100", but is watchable. At 100", you can really see the difference between 720p and 1080p, so SD stuff is on a whole other level.
I would say of all you have is an SD source, it's probably not worth it.
I can't help with SD from Virgin Box but I can say that it shows a massive improvement on SD via DVD from my previous projector, Benq w100, with the increased resolution. In fact it's the first time I've ever seen a noticable difference with upscaling as my player upscales to 1080p it sits well with the projector. Mybe you have the option of being able to upscale, via an upscaling Receiver or other?
Some SD DVDs look pretty close to Blu Rays. Obviously not all DVD and not not all Blu Rays are equal but the projector definately performs