I am not trying to bash raven or anything, but just telling the guy its pointless doesnt help him. Give him some advice, explanations or suggestions. With that said I did some quick research using google on your specific situation. I came across this website article:
http://www.theinquirer.net/default.aspx?article=32139
I am going to specificly quote some frame rates they got for BF2 using the 7900GT:
Low quality
Single 7900GT @ 2048 X 1536 FPS: 41.55
SLI 7900GT @ 2048 X 1536 FPS: 69.17
High quality
Single 7900GT @ 2048 X 1536 FPS: 44.78
SLI 7900GT @ 2048 X 1536 FPS: 73.12
In my opinion those numbers tell me that playing battlefield 2 with SLI 7900GTs is far from pointless. That is a huge framerate jump and anything around 60fps in my opinion is beautiful at that resolution.
If you were to play at the lower resolution of 1024 x 768 your really not going to benefit from SLI (and maybe lose a little frame rate it seems) but from what you told us is that you going 1980 x 1080 if that is the case im going to have to go against what everyone else is saying and tell you that SLI WILL make the difference and I actually have numbers that show it. But again I only have one source for these numbers and I am not going to take the time to search the internet... and I may be wrong, but atleast I provided you with more information then the others.
Conclusion:
7800GTX is a badass card, if you want the SLI 7900GTs your going to notice a difference at high resolutions and high quality, I only have evidence to back that up. Until someone can come up with a good reason to believe those numbers are wrong, or that I am mistaken about something, I see no reason to not get the two 7900GTs and play BF2 in all its glory! Now if it was me I would be happy with the 7800GTX and try to maybe OC it a bit or just wait to do a whole computer upgrade when DX10 cards and vista become available. Either way good luck with your decision and hopefully the people on these forums can come up with more then a half-assed answer for you and provide you with good advice!