I do know they had to be landed, male and Roman citizens

They didn't, actually. Or rather suffrage was extended to all adult male citizens regardless of their property--the capite censi, the lowest century of the comitia centuriata consisted entirely of the unpropertied. It's just that the votes of the capite censi (which altogether numbered only a single century, pretty much a totally irrelevant vote), and the nearly-landless lowest property class in general counted for next to nothing. In the comitia tributa, however, voting classes were separated by district, not wealth.

The vote in Roman politics took place exclusively in the city. To participate in the vote a voter had to be physically present to confer with his century and cast his tablet. If a citizen lived in a different part of Italy he had to physically travel to Rome in order to cast his ballot. However, this does not in fact mean that citizens inhabiting rural districts were particularly poorly-represented. Since the vast majority of the urban population was either unpropertied or in the lower property classes it was the rural voters who voted first and whose votes counted more. The rural plebs generally held some small amount of land, or rented it, and at times their electoral presence was the critical factor of an election, although usually the comitia centuriata was dominated by the first property class, and in particular the senatorial sex suffragia. The comitia tributa was somewhat different, however. Only four of the thirty-five voting tribes were urban, meaning that even if there were more total voters in the urban tribes the rural tribes could easily outvote them, as each tribe's vote only counted once. Now the rural tribes were often represented by the landed senatorial class or equites, whose landholdings enabled them to maintain themselves in rural tribes, but the entry of the rural plebs into the city for the tribal vote could be decisive. Tiberius Gracchus' land distributions were passed because he called on the rural plebs to assist him at the vote, and Appian blames the failure of his bid for reelection to the tribunate on the fact that the tribunate elections took place during the harvest, preventing his rural supporters from traveling to the city to vote.

There's something of an ongoing debate in the field as to the degree to which the Republic was truly "democratic." Fergus Millar argued for widespread popular participation in the political process, but the general trend these days is to follow more closely Mouritsen, who laid out economic and geographic limitations on the possibility of general popular participation. The urban plebs and rural plebs both had to work, and work quite hard--for the average urban day-worker, who made up most of the urban population and who risked starvation if he missed a day's work, forgoing a day's work to vote might be a tragedy in the making for his family. And why would he, when his vote counted for next to nothing? He probably wouldn't even get to vote in the comitia centuriata, since his vote would be last, and even if the four urban tribes got the chance to vote in the comitia tributa (slightly more likely, but probably not common) his tribe's vote was almost totally worthless. The average rural farmer, whether he owned his own little plot or rented it, could afford to leave his fields for a time during certain parts of the year, but it was an undertaking that might not be worth the effort. There are lots of other considerations, like the size of the forum, which almost certainly couldn't house a particularly sizable proportion of the full citizen body (although I think, having seen the forum for myself and being astonished by how many tourists can pack themselves into it, that Mouritsen's estimates are too low), but for simplicity's sake let's leave it there. It's somewhat hard to say from where and from what social levels the Roman electorate hailed. One thing we can say pretty definitely is that the senatorial and equestrian vote dominated the comitia. But even though voters had to physically travel to the city to vote we know that at least sometimes significant numbers actually did, and Badian argues for an Italian equestrian clique in the years following the Social War that might not have had many ties to the city itself (their actual homes were probably elsewhere in Italy, unlike the senatorial class or the Roman equites who lived in the city but were registered elsewhere) but entered the city to support Marius and Sulpicius. It's sort of a complicated question. There were also local elections and votes in many Italian (and provincial) towns, which in certain circumstances might be actually be more important to local voters than the Roman vote