I have again run into a dilemma, and have a question that you genious' can hopefully answer (I'm still new to computer science). I have an Asus M4A87TD mobo and it has 2 PCI 2.0 express slots for gpu's but it isn't corssfire or sli supported. I personally don't understand the point of the extra slot if you can't do it, but I'd thought I'd check first. Could I buy another GTS450 and link it with SLI? Or even if I don't link it will I see a performance boost and let it take care of physX or something?
You can crossfire on your mobo (crossfire/SLI ready tag is just a marketing gimmick).
The only problem that you will encounter was best described by serge44
'2. I would get a MB that can at least run them on 8x8.. 16x4 will definitely limit the bandwith for the one running on 4x, while 8x wouldn't be a problem. It's not that anything bad is going to happen, it's just that you wouldn't be getting all the juice out of your second card as you should. '
It appears I am mistaken about multi gpu setups on that board. Though i'd like to point out in yamman101's link, they did testing with gtx 570s in that post. A gts450 would not be bottlenecked by a pci-e 2.0 x4 slot.