Newbie needs advice

[alopez65]

hello, i'm new to this forum and i'm enjoying all the info you guys share. i didn't think i was such a newb at this stuff, but i am humbled by everything i'm reading within this site.
i need some advice. i had a rptv and upgraded to a sammy 58" plasma and just bought a blue ray player. i was using a htib and wanted to slowly upgrade because of budget. i purchased a sony receiver and a set of yamaha speakers from ebay and i found out they aren't compatible. the receiver doesn't have any rca outputs for a power sub and the power sub only has a single rca input.
my question - which should i save for- a new receiver or a different sub. i don't mind good used equipment due to budget constraints, but i won't spend more than a couple hundred for one or the other. thanks in advance for any help.
speakers - yamaha ns-sp1800
reciever - sony str-kg700