sorry, but for bootscreen there is an aftermarket on ebay. there is also a seller from australia offering cards there (had bought from him in the past once), maybe you could get in touch with him? the 7300gt has no hdcp, is not future-proof and seems to sorted out by you (due to non-compatibility with teh r9 280(x) in bootcamp); the x1900 is single slot, too, but i consider it poor option (seems loud); the 2600xt seems very viable (and costs also quite alot on ebay).
i have actually bought an r9 280 for myself , too ; but after finding out that the drivers package to run 7950/70 or 29 280(x) on lion 10.7.5 are not compatible with 32.bit efi (on 1.1/2.1 MP the R9 280 needs minimum 10.8.3+tiamo bootloader as it seems) i decided to sell it as it would limit my options on booting osx, and i don't like to change cables every time i want to boot into another system.
possibly whenever you want to use geforce and radeon at the same time in windows, there might arise problems?
in believe that all readers of this forum have upgraded to 10.10.5 and 10.11.6 neither of which are supported by apple, and i am using a R9 280x not flashed which is also not supported by apple.
i believe there is a solution, but which radeon card is appropriate i have yet to discover.