It might be worth considering putting a second 9800GTX+ in SLI with your current card. I'm not sure if that would give you quite the performance boost that you're looking for, but it would be fairly cost effective and allow you to keep your current card.
Right now I'd say stick with your current rig and upgrade if needed, there's no point in spending money in these cash-strapped times when you do n't need to!
As for SLI, which MB do you have? Only Nvidia chipsets support SLI for the LGA775 chips.
Not every game uses SLI, so check the AION website carefully, posting questions if
you have to to find out. If the game does not benefit from multiple cards, getting another 9800 would be an obvious waste of money.
Given your specs and budget I'd say the best overall upgrade would be a GTX275, it's a little cooler and quieter than the HD4890 and you do n't run the risk of driver problems by sticking with the same manufacturer. They can be found on the Egg for less than a paltry $220
personaly i would sell the 9800 and buy the 5870 ati when they come out
no point in getting two old cards
it has better shadows
directx 11 ready
512-bit bus and the tried and tested GDDR5 memory
&twice the ram as your 1 at the mo