The LTCG on the sale of your flat bought in 1998 will have to be worked out with reference to the indexed cost of acquisition of the flat. The indexed cost of acquisition of is roughly 3 times if the asset is sold in the current financial year. The amount of capital gain is exempt u/s 54, if it is invested in acquisition of another residential house, subject to the fulfillment of conditions specified therein. Or if the sale proceeds are invested in capital gains bonds upto Rs 50 L, the entire capital gains will be exempt u/s 54EC, subject to the fulfillment of conditions specified therein.
You need to balance out your cash flows and tax liability based upon the facts of your case,by consulting a Chartered Accountant in your area.