Acurus Act 3 vs Lexicon Processor

The Act 3, unfortunately, stock, ISN'T a dead quite unit, no. It is a tad noisy, but not as noisy as others. It's got about a 90db signal to noise. Others I've had are around 96 or higher...which is better.
I've never had a problem at all with the Act 3's I've owned, and would continue to use them in a pinch for cheaper, quality/superb sounding 5.1 pre's. I wouldn't however own ANY LEXICON! I've sold em over the years, heard em in many many systems, and they're very "plain vanilla" digital sounding pre's IMO. Not so musical, a bit sterile, and I definitely wouldn't use em in a high end 2 channel system on their own, that's for certain! Again, this is my experience.
If you simply want "wiz-bang" surround stearing and processing, and overall quality of sound isn't so critical, the Lex is good. If you need more refined sound, even the old ACT 3 punishes the sound quality of the Lexicon stuff! I say this from years of experience with both brands.
I have an ACT 3 that was moded by the factory however, that's extrememly quite, if almost dead quite from the speakers! It's upgraded to 24/192 dac's and such, 7.1 channel pass through, direct analog in's for all inputs, bass managment, DPLII (at least as good as Logic 7 on Lex). But then something like that's a bit more money used.
However, if you need dead quiet, you'r leaning more towards the Lexicon between that and the standard stock Act 3. Personally, I'd look elsewere

"Personally, I'd look elsewhere?" Where would you look. Again, I only need the processor for HT, music is handled elsewhere.

At $500? That is tough. The act 3 is a veritable bargain in this price range. But it isn't the quietest in comparison to some. So that's probably out.
I'd probably consider used Thule 5.1 dd/dts piece. The Outlaw 950 is good sounding for the money for certain, but a little shy on dynamics IME.
Actually, dare I say it, but a used receiver from Yamaha makes for a good AV pre/pro for movies. Anyway, seems to be detailed enough (most better than, say, a Classe SSP25), and rather dynamic used as a pre/pro. For cheep some of these more recent pieces are worth a try.
Remember, for $500, you don't have much choice. Still, not bad

If it is only for movies, get a Rotel 1068 and save a bunch of cash, it has great adjustments for each speaker, bass management and individual tone controls for each speaker is IMO a really nove feature.
I know there are alot more expensive units that MAY be better....but if you not anal about a simple movie system the Rotel is a great buy. And the opinion on the Lexicon by the other poster is simply opinion....the Lexicon can and does sound very good, and I have alot of experience with my fathers unit.

If you really want to save a lot of money, and you're doing this only for movies - here are some more thoughts:

The B&K AVP3090 is a steal on the used market at <$300, and somtimes close to $200. It sounds great and is reasonably quiet, although may be suseptible to RFI that requires special attention. You can probably get a Ref 20 for under $500, and Ref 30 maybe for a tad more than $500 depending on condition/age. B&K service and support is the best I've experienced from any company. The 3090 doesn't have optical in, but does support DD and DTS.

I don't have experience with the Rotel 1068, but do with other Rotel pre's and it's been good. They're not bullet-proof like B&K, but they are built nicely, and sound great.

The Sherwood-Newcastle AVP9080r can be had for $200 and is quiet. Setup isn't as user friendly or flexible as the B&K nor the Lexicon DC-1. One strength is it has 5.1 inputs.

I tried several Yamaha receivers before finally going to a separate pre/pro, and amp. Don't waste your time and money on them.

I'm not sure if this is a fair comparison, but I had the Acurus Act 1, which is pro-logic only, and while for the money it was nice, it was very noisy and prone to ground loop. It's also a bit bright. I don't know of these traits are common to the Act 3.

Back to the Lexicon DC-1, I went throuh all of the above, still have the 3090, and the Sherwood, and the DCs is my primary HT processor. I frankly can't relate to the comments made above. The sound quality rivals many pure stereo pre's I've tried, including Rotel RC1070, and also Classe CP35. It has its shortcomings, but I really like the flexibility (7.1 capability) 7.1 inputs, Coax and Optical ins (though only 2 of each), and easy, flexible setup. My use is PRIMARILY music videos in a dedicated home theater, so the music quality is important to me. I generally watch movies upstairs in a less critical environment (ie the family room) using the B&K as a pre-pro.

Finally, since you're talking about used here - I'm sure it's not gotten by you that you can buy one, if you dont like it resell it and move on to the next one.