Cool. I just picked up a floor model Pioneer 1014 from Best Buy a few days ago to power my
Onix Rockets, but that's probably a little more than you're wanting to spend on speakers.
First of all, if you haven't checked out AVSforum or hometheaterforum, I would highly suggest going over there too to get some more advice.
Ideally you'd want all 5 speakers in your HT system to be from the same company and the same series of speakers so that they would all be timbre matched and panning effects wouldn't sound strange because all the speakers would have the same sonic characteristics.
The surrounds don't matter much for this though, because most panning effects are a left to right or right to left motion so the matching for the front 3 is more important.
It's not a huge issue though, so if you want to keep your current speakers, that's fine.
A good center channel is very important for DVDs. Something like 70% to 80% of the sound in a DVD movie is coming out the center channel.
Off the top of my head, a great deal on some speakers that are being discontinued (I think) from Onix (the company that I just bought my own speakers from) is happening.
You can get a set of
RS150s in ebony and an
RSC100MKII (probably in ebony too to match) for $400 + shipping. I think these are about half their normal price right now and were a good deal to start with.
The guys at AV123 are great. If you have any questions about their stuff or HT in general, they're always willing to help.
At those prices, I think it's going to be hard to get a better 3 speaker set. It's such a good deal that you might want to think about getting a whole 5.1 set and replace your 30 year old speakers. Maybe order the first three and then do a comparison (with a sub to cover the low end) between the RS150s and your current speakers. If you like the sound of the RS150s better, you could pick up a second pair and make a matched 5.0 set for yourself (just a thought

).
Alternatively, if you just want something to produce sound,
justdeals sells 3 speakers from a z-680 set for $35.
There are lots of options in between too
If you can go to an audio store and demo some speakers to see what kind sounds good to you, that would be a good idea too. I've been told that the higher end Rockets sound similar to the Paradigm Studio series. I'm not sure about the lower end Rockets.
For a sub, there are a lot of options too.
A good subwoofer adds a lot to movies. After the center, I'd say a sub is tied for importance with the front stereo speakers.
For something along the lines of the Logitech prices, I'd recommend the
dayton 10" from partsexpress. It's a nice sub that goes pretty deep and served me well while I had it. Until you get into the $250-ish range, it's really hard to beat that dayton.
So, once you get to the $250-ish range, there are some other good options (Like the JBL e150), but at that point it's almost better to just move up to the $400-ish level and get what many call the "entry level HT subs".
At the $400-ish level, some really good options are the
SVS PB10 (I own that one currently) and the
HSU STF-2.
I'm not sure if any of these suggestions would fit into your "fair amount" criteria, but it's a start. I've been reading up on a lot of different products up into about the $3000 range for a set of speakers so I have a good idea of what's good for the money.
I'm not an expert, but I can point you in the right direction if you have any other questions

.