I have a basic RSS aggregator running on my OpenACS 3.2.5 site. I've worked on it off and on (you're welcome to look at the somewhat patchy code if you'd like). I went straight to XSLT for converting RSS to XHTML (different transformation depending on what version of RSS the feed advertises itself as / if it's RDF). As I use conditional GETs and caching of the XSLT transformations, I find this doesn't present much of a resource problem (then again, I don't get much traffic - I'm the only user of my aggregator :). Of course, you have to depend on the producer of the feed to generate valid XML; I found a couple choice regsub's corrected most errors.
By the way, I also found I had to modify AOLserver's http.tcl to retrieve response headers out of ns_httpget (and make it understand HTTP 301 and 304), namely accessing ETag or Last-Modified information from an RSS feed. I didn't use util_httpget, but from the API it looks like it suffers from the same response headers weakness.