The Witcher 3 doesn't actually use up a lot of memory, it barely reaches 1.5GB usage on my 7870 XT on with textures on ultra and most settings on ultra except shadows on high, view distance on medium, and SSAO running at 900p. So I'm guessing that at 1080p, it will be using 1.5-2GB. Nevertheless, the R9 290 with its Windforce cooler is much better than the 780. Even if they were both reference actually, the 290 will still perform better.
Here is a comparison from Anandtech of the 2 cards:
GTX 780 vs R9 290.
AMD also just released its first WHQL driver for 2015 last July which brings a lot of fixes and performance increase for its GCN-based cards.
edit:
saw your follow-up question and I actually owned a USP 100 before (I live in the Philippines) and it does not support very long graphics cards since the space where hard drives are installed extends to the bottom of the case and aren't removable without using tools. Looking at the specifications, it MIGHT JUST fit the R9 290 Windforce since the card is 293mm long while the USP 100 officially supports graphics cards up to 290mm. The card also has its power connectors located on the side rather than some which place the connectors at the back, giving you a better chance of fitting the 293mm long card.