I think the best gains for the new brewer is getting to all grain (as long as they have the time and space). The equipment needed is minimal and it lowers the cost of your beer and opens up a world of new items to play with (grains).
A 10 to 15 gallon pot with a ball valve and a 10 gallon cooler with a ball valve and stainless steel braid are all you need.
*sigh*
[snipped for space]
And it's just silly and plain tiring to see this stuff. Not every extract brewer wants nor desires to go to AG, just like not every bottler dreams of kegging, or every homebrewer dreams of going pro.
Oy vey...
As much as I agree with you Revvy, the OP did mention going all grain in another 3-4 batches, so at least the second half of DonMagee's post was not completely out of line. It was as least as helpful as the "Go with BIAB" reply #1.
I still believe that getting good, solid fermentation temp control in place comes way ahead of making that leap.
This is exactly where I would spend the money, as I feel it would be the most useful right away and long term.
Yes, the OP will need a larger pot when they finally go all grain or even full boil extract, but that may be another 3-4 batches like he said, or it may be a number of batches down the road. Regardless of when/if they go all grain, they will still get the benefit of fermentation temperature control while continuing to make batches with extract.
After temp control, I would probably go with the simple equipment to make yeast starters, just because it is inexpensive and helps to make better beer. You can make a starter with a jar/bottle sitting on the kitchen counter and swirling it every so often. It doesn't work as well as a stir plate, but it is usually better than simply pitching a single vial/smack pack. Stir plates are fairly easy to DIY as well.