To SLI or not to? Need advice

Hi,

So i finally joined these forums after reading so much great info and advice and now i need some of my own. I am currently upgrading my PC which I use to play games, mainly TF2, BF3 and Batman Arkham City.

Now so far I have bought a Phenom II x4 965 Black Edition at 3.4GHz, a chunky gaming case with lots of internal fans, 8GB of RAM at 1600MHz. I currently have filling the gaps a 500W power supply, pretty cheapy/standard ASrock motherboard and an Nvidia GTS 250 OC 512mb along with a harddrive that I dont intend on changing.

Now what I advice on is a dilemma I cant come to a decision upon, whether or not to SLI my current graphics card due to my friend having the exact same card in good working condition willing to sell to me for a good price. Now, if I choose to SLI I will have to upgrade my motherboard (I need to anyway to accomodate the new CPU) to a 2 xPCIe one that I have found for £50, I will also undoubtedly need to upgrade my PSU to atleast 750W? Not to sure, and obviously the other option is to sell my current GTS 250 and go ahead with buying a new GFX card for around £100-130. This way the two options pan out to approximately the same cost but which one will be better for my needs?

I understand that TF2 and other source engined games dont support SLI which is a bit of a bummer but my main ambition for this upgrade will be to have BF3 running a med-high graphics at GOOD fps (60fps+ 99% of the time) and at a comfortable fps if i chose to record, with eg. fraps.

Basically, I need some opinions on what will get me the best outcome. Maybe suggest anygive new cards that will give me a decent performance.