Can anybody help with upgrading?

A couple of months ago i bought myself a new computer, at the time i had no knowledge of computers or about graphics cards or ram, after realising this emachine was quite crappy i decided its time for some upgrading., first of all i added 2 gigs of ddr2 ram, which helped my gaming alot, but not everything, so i decided, ill get myself a 9800 gt graphics card, but to my dismay i found that my computer only has a power suply of 250 watts..., any advise is welcome ( thanks in advance )