To answer your main question, generally large OEMs tend have better deals right before school starts, and right before Christmas, though they tend to have special coupons and deals at fairly random times. Check their websites.
Now onto the next question!
First of all, do you plan on buying a machine from a large OEM(Dell, Gateway,ect), having it built by a small whitebox builder, or are you planning on building it yourself? For the best price/performance/quality, you want to build it yourself, followed by a whitebox small company. Large OEMs can sometimes have good deals, especially in the back-school time period of late August, and are hard to beat on the low end. But remember, they tend to use shoddy parts whenever they can get away with it - especially with things like the Power Supply, no-name brand RAM, or a cheap motherboard, since most of their customers are ignorant of how important those things are.
Secondly, what do you want to use this computer for? Generally, for gaming machines, AMD’s Athlons (especially the Athlon64s) are better than Intel’s Pentium4s. And with a gaming machine, you want to get a high end video card - the video card is THE most important part of a gaming machine. But for video editing and media encoding - Pentiums 4s have the speed advantage there. Of course, if all you do is web browsing/email/word processing/IM, ect, than any modern processor is more than fast enough.
Right now, for the best price/performance for a lower-mid range machine I would go with an AthlonXP processors - at the lower price range, they are competing with Intel Celerons, which simply can’t keep up. Heck, a 1.6ghz Duron (Durons are crippled budget versions of the AthlonXP, much like the Celeron is a crippled Pentium 4), will beat a 2.6ghz Celeron in most benchmarks.
As for video cards, while ATI and Nvidia each just released a new series of cards (Radeon X800 and Geforce 6800 ) they are still very expensive. You best bet is go with with the last generation of Radeon cards - the 9800 Pros are very powerful, and are practically a steal these days, dropping down the the $200 mark. Radeon 9600XTs and Pros are also very good deals. Generally, I would stay away from the GeforceFX cards - they are fast in older games, but have performance issues in newer DX9 titles.
As for RAM - get at least 512MB, PC3200 - Corsair and Crucial are my prefered brands, Muskin isn’t bad either.
Hard drives - Seagate, Maxtor and Western digital are all good, make sure to get an 8mb cache version - much smoother.