With the stock JG Bar-10 w/ Scope Package, all of the DIY mods, and a Nineball bucking (i guess Firefly if you prefer), you could potentially get 500 fps with very good groupings at 150'. That will cost no more than 130 dollars for everything. You next move should be a tightbore, if you would be willing to through in another 50-80 dollars.
[Teflon mods are explained well in The Crimson Falcon's Comlete Guide to Sniping under consolidated Stickies. Look there, then see if you have questions...]
At 500+ fps I would recommend using BIOVAL's washed. Do not using anything less than .27g BB's. Quality BB's are probably the most important accuracy upgrade, and using .12g low quality BB's, you couldn't get good groupings from 75'. You may even want to go for some .36g or so. Look into ammo a little more closely (there are plenty of threads around).
If your budget is 120-130 with the gun, this is by far the best recommendation. When I got my stock bar and tuned it and everything, even at 150 with the stock bucking I only got 5/10 hits on torso sized target. Once I got my nineball bucking my hits were up to 8/10 or 9/10 hits at 150' on a torso sized target on a calm day.