Some people post a lower range for their budget. I'm the king of typos so I asked to be sure.
For less than $500, there is alot you can do. If you can keep the speakers and deck for now, then you might get an integrated unit. Jolida makes an integrated hybrid unit (tubes in the pre and a solid state amplifier stage) the sells for about that price. There is a 1501RC on audiogon right now for $550. Those occassionally come up for $500-$450. I forget which which model but there is one with lower power output that usually sells used for around $400. You can get something for less from Rotel or NAD. If you prefer seperates, you can get a decent amp and preamp combo for that price. The B&K ST1400 or ST202 will run you about half your budget leaving the rest for a preamp.
Are you interested in something with tubes? What kind of speakers do you have? To what kind of music do you listen? Do you plan to eventually build out an audiophile 2 channel system or are you more interested in multi channel? All these things will affect the list of good options. If you plan to build a high end system down the road, you should be selecting components today based on an upgrade strategy that allows you the most options. For example, either of those B&K amps will be good enough to last you through at least a couple of preamp upgrades and they have enough power to drive some planars if you decide to go that route with speakers.
Look through the posts in this section and you'll find a lot of equipment reviews - not all of them are for multi-thousand dollar components. There are some really good systems people have put together for a reasonable budget. Also you should look on audiogon and see what amps, preamps, or integrateds you can afford. Then search the agon discussion forums to learn about the individual pieces. Sites like audiocircles and audioasylum can yield a lot of information. Take a look though the audiogon and audioasylum user system descriptions and see what budget and second system components people have paired - read their reviews.
http://forum.audiogon.com/cgi-bin/frv.pl