On games that support them, the performance difference is awesome. Two 275 SLI will be faster than one 295. However on games that have little to no multi card support, the performance gain is largely, if not, neglible. In rare cases two cards can actually hurt performance.
Graphial glitches. Games and drivers aren't perfect. Neither is the support. The majoritty of the time they are just little nusiances, at worse extremely annoying.
Is your board a 35/45/ or 38/48 ? A single 295 will run at x16 while two 275's may run at x8/x8. However it's debatable if there is any noticeable difference.
Check your PSU can handle either a singe 295 or two 275's to begin with.