5970 in "Quad" is the best. Its acutally just Crossfire, and your 295's are just SLI.
5970 CF performs the best out of them.
To quote the ranking list
01. Geforce GTX 480 In Quad SLI
02. Radeon Ares HD5870X2 In Crossfire
03. Radeon HD5870 In Quadfire
04. Radeon HD5970 In Crossfire
05. Radeon HD5850 In Quadfire
06. Geforce GTX 285 In Quad SLI
07. Radeon HD4890 In Quadfire
08. Radeon HD5830 In Quadfire
09. Geforce GTX 480 In Tri SLI
10. Radeon HD5870 In Tri fire
11. Geforce GTX 470 In Tri SLI
12. Radeon HD5850 In Tri fire
13. Geforce GTX 295 In SLI
HD 5970 CF doesn't scale very well because of drivers, currently neither quad SLI/CF perform scale well compared to 2-way SLI/CF.
However since you already have 2 GTX 295S,the only options that will give you a boost over your setup would be either 2 5970s or 2 GTX 480s.