SLi vs Crossfire, about same really, all depends on the drivers but you will get good performance with either Nvida SLI or ATi Crossifre.
The GTX280 is better then the 4870 1gb yes but 2 4870 1gb's in crossfire is better then a GTX280 and you want to save money and be more future proof so maybe just get the 4870 1gb and then buy another one next year or whenever you need the extra performance.
Some mobo's here's:
Sli:
http://www.ebuyer.com/product/142585
http://www.ebuyer.com/product/139415
CF:
http://www.ebuyer.com/product/145751
http://www.microdirect.co.uk/(29126)Gigabyte-motherboard-GA-X38DS4--LGA775-Intel.aspx
The GTX280 is better then the 4870 1gb yes but 2 4870 1gb's in crossfire is better then a GTX280 and you want to save money and be more future proof so maybe just get the 4870 1gb and then buy another one next year or whenever you need the extra performance.
Some mobo's here's:
Sli:
http://www.ebuyer.com/product/142585
http://www.ebuyer.com/product/139415
CF:
http://www.ebuyer.com/product/145751
http://www.microdirect.co.uk/(29126)Gigabyte-motherboard-GA-X38DS4--LGA775-Intel.aspx