The r7 250 is a really weak gpu. When they are in crossfire, they may give a quite decent performance, but I'd suggest getting an R9 280 or 280x. The reason is that multi GPU systems are a nightmare to use, plus only mantle, dx10 and dx11 have a full support for multi GPU. While DX9 is quite bad (high FPS spikes) and those that can't even use multi GPU basically run on one card.