10.6 Snow Leopard would be excellent choice for your 2008 MBP, what it does is strips out the PPC code your not using in 10.5 Leopard and offer faster video drivers and some nice bells and whistles.
10.6 is faster than 10.5 (video driver improvements) and 10.6 is slightly faster than 10.7.1 (tested on the same hardware)
(It might be that 10.7.3+ is possibly a hair bit faster than 10.6, I haven't seen any recent speed comparisons lately.)
A advantage with 10.6 is you get to keep running most of your present 10.5 software, some will require a update to work in 10.6 under Rosetta which is a compatability layer.
Most all software and hardware drivers for older third party devices work for 10.6, so your good to go.
Do backup your files off the computer to a storage drive (not TimeMachine) and disconnect before making any major changes.
You can buy the $29 10.6.3 Snow Leopard upgrade/install disk from Apple online.
http://store.apple.com/us/product/MC573Z/A
(to boot off the 10.6.3. disk, hold c upon boot. You then have two choices, upgrade 10.5 to 10.6 only, or use Disk Utility first to erase everything on the drive then install 10.6)
Now about 10.7, it's a new OS that came out in July, it's all 64bit and thus it doesn't run Rosetta (of 10.6) and thus your 10.5 software will not work with it.
Some third party software is not ready for 10.7 yet, and a lot of hardware drivers for printers and scanners may not function for it. You will have to do your homework check this table.
http://roaringapps.com/apps:table
10.7 Lion represents a rather drastic change to OS X in some aspects that need getting used to, no boot disks, internet recovery and the MacAppStore etc.
10.8 is going to be released THIS SUMMER and nearly no software is ready for it obviously and won't run on older hardware (to be determined)
I think 10.7 should run fine on your machine, you may have to upgrade the RAM to 4GB to get better performance.
So you have to make a decision, if it's wise for you to have to buy all new versions of software for your older Mac and run 10.7 or stay with 10.6.8 and run the software you have (minor updates) until perhaps a year from now when 10.8 is out on brand new hardware and the bugs are worked out of it, buy all new software then.
IMO option #2 with 10.6.8 now, 10.8 later on new hardware is a better choice than option #1 going with 10.7 and paying more money for software for little reason, or going without as some software isn't ready yet for 10.7.
I don't know what or how much investment in software you have; if little, then 10.7 is nothing, if a lot then you perhaps may want to hold off 10.7 for now.
If you do decide to go to 10.7, I advise buying the $69 Lion USB thumb drive and option key booting from that, erasing the entire drive and installing 10.7 directly. Skipping right over 10.6.
With the Lion USB you can install 10.7 anytime you want, no internet connection required, however if you upgrade 10.5 to 10.6 ($29) then to 10.7 ($29), then you have to use Internet Recovery from your boot drive, I don't know how reliable your Internet is, but a $69 USB is certainly more reliable for only a few dollars more.
Good luck.
