If you feel the need for graduated ND filters, then you need Photoshop. Elements is fine for now. You can use it to process your RAW files and blend together different exposures or even use the same exposure to accomplish the same goal (fit a too-wide luminance range into a printable image.) Learn the proper techniques and you'll get much better results than the ND filters. (I ditched mine years ago.)
So that's $80 spent.
The rest should go into a better tripod. My reasoning is the same as above -- if you are shooting things that require graduated ND filters, then you are shooting things that require a good tripod. If you want to be able to blend exposures, you need a good tripod. And so on.
While a high-end carbon fiber tripod is great, your budget probably won't go that far. You can pick up a decent aluminum Bogen tripod for less than $150 (legs only), and add a decent tripod head like the Acratech Ultimate Ballhead for about $300. Neither of these is the greatest on the market, but they are both good choices. Others might suggest different models.
With shipping and tax and maybe a quick release plate or two, there goes your $600.
Next up, when you have a few more bucks, you can look at replacing your 18-55 with the IS version for less than $200. Or possibly some other lens choices.
And hey, we all have a budget, and fully understand.